Use named subject in RSpec tests

## What

Use named subject, [doc excerpt](https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#subject-and-let-variables):

> Avoid referencing subject in examples. Use a named subject subject(:name), or a let variable instead, so the variable has a contextual name.

## Steps

1. Enable Rubocop rule and exclude existing offenses - https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/3388
1. Fix all other [places](#existing-offenses-to-fix)

### Existing offenses to fix
- [x] qa/spec/unit/helper/zuora_client_spec.rb
- [x] spec/controllers/api/countries_controller_spec.rb
- [x] spec/controllers/api/customers_controller_spec.rb
- [x] spec/controllers/concerns/simple_encryption_spec.rb
- [x] spec/controllers/eula_requests_controller_spec.rb
- [x] spec/controllers/instance_review_controller_spec.rb
- [x] spec/controllers/instance_review_settings_controller_spec.rb
- [x] spec/controllers/invoices_controller_spec.rb
- [x] spec/controllers/orders_controller_spec.rb
- [x] spec/controllers/payment_methods_controller_spec.rb
- [x] spec/controllers/zuora_callbacks_controller_spec.rb
- [x] spec/features/rails_admin/list_gitlab_namespaces_spec.rb
- [x] spec/finders/invoices_finder_spec.rb
- [ ] spec/finders/license_finder_spec.rb (the class doesn't seem to be used anymore)  - !4161
- [x] spec/finders/order_finder_spec.rb
- [x] spec/finders/subscriptions_finders_spec.rb
- [x] spec/graphql/mutations/cloud_activations/activate_spec.rb
- [x] spec/graphql/types/plan_tag_type_spec.rb
- [x] spec/graphql/types/subscription_type_spec.rb
- [x] spec/helpers/registrations_helper_spec.rb
- [x] spec/helpers/subscription_upgrades_helper_spec.rb
- [x] spec/jobs/application_job_spec.rb
- [x] spec/jobs/billing_rate_update_cron_job_spec.rb
- [x] spec/jobs/billing_rate_update_job_spec.rb
- [x] spec/jobs/expire_trial_cron_job_spec.rb
- [x] spec/jobs/gitlab/sync_order_job_spec.rb
- [x] spec/jobs/send_upcoming_reconciliation_email_job_spec.rb
- [x] spec/jobs/zuora/actions/amend_spec.rb
- [x] spec/jobs/zuora/actions/delete_spec.rb
- [x] spec/jobs/zuora/actions/subscribe_spec.rb
- [x] spec/jobs/zuora/actions/update_spec.rb
- [x] spec/lib/admin_builder_spec.rb - !4088
- [x] spec/lib/client/license_app/license_data_spec.rb - !4051 
- [x] spec/lib/client/license_app_spec.rb - !4088
- [x] spec/lib/country_spec.rb - !4088
- [x] spec/lib/errors_spec.rb - !4088
- [x] spec/lib/gem_extensions/iron_bank/local_spec.rb - !4088
- [x] spec/lib/gitlab/http_spec.rb - !4088
- [x] spec/lib/gitlab/tracking_spec.rb - !4088
- [x] spec/lib/gitlab_danger_spec.rb - !4088
- [x] spec/lib/health_check/custom_spec.rb - !4088
- [x] spec/lib/middleware/health_check/health_check_responder_spec.rb - !4088
- [x] spec/lib/middleware/zuora_proxy_spec.rb - !4088
- [x] spec/lib/salesforce/attribute_builder_spec.rb - !4088
- [x] spec/lib/salesforce/object_type_mapper_spec.rb - !4088
- [x] spec/lib/subscription_current_version_finder_spec.rb - !4088
- [x] spec/lib/subscription_params_generator_spec.rb - !4088
- [x] spec/lib/utils/escape_spec.rb - !4088
- [x] spec/lib/utils/strong_memoize_spec.rb - !4088
- [x] spec/lib/validators/subscription_validator_spec.rb - !4088
- [x] spec/lib/world_spec.rb - !4088
- [x] spec/lib/zuora/associations_spec.rb - !3867
- [x] spec/lib/zuora/digital_signature_spec.rb - !3867
- [x] spec/lib/zuora/fields_spec.rb - !3867
- [x] spec/lib/zuora/query_builder_extension_spec.rb - !3867
- [x] spec/lib/zuora/resource_spec.rb - !3867
- [x] spec/lib/zuora/subscription_spec.rb - !3867
- [x] spec/mailers/admin_mailer_spec.rb - !3866
- [x] spec/mailers/custom_devise_mailer_spec.rb - !3866
- [x] spec/mailers/customer_mailer_spec.rb - !3866
- [x] spec/mailers/gitlab_internal_mailer_spec.rb - !3866
- [x] spec/models/amendment_spec.rb - !4080
- [x] spec/models/customer_spec.rb - !4080
- [x] spec/models/instance_review_setting_spec.rb - !4080
- [x] spec/models/instance_review_spec.rb - !4080
- [x] spec/models/license_seat_link_spec.rb - !4080
- [x] spec/models/order_spec.rb - !4080
- [x] spec/models/plan_spec.rb - !4080
- [x] spec/models/plans_finder_spec.rb - !4080
- [x] spec/models/reconciliation_spec.rb - !4080
- [x] spec/models/subscription_spec.rb - !4080
- [x] spec/models/trial_user_spec.rb - !4080
- [x] spec/presenters/customer_presenter_spec.rb - !4020
- [x] spec/presenters/order_renewal_presenter_spec.rb - !4020
- [x] spec/presenters/payment_method_presenter_spec.rb - !4020
- [x] spec/presenters/payment_methods_collection_presenter_spec.rb - !4020
- [x] spec/presenters/plan_upgrade_presenter_spec.rb - !4020
- [x] spec/presenters/product_presenter_spec.rb - !4020
- [x] spec/presenters/subscription_license_presenter_spec.rb - !4020
- [x] spec/requests/marketo_request_spec.rb - !3866
- [x] spec/serializers/gitlab_plan_serializer_spec.rb - !3867
- [x] spec/serializers/subscription_serializer_spec.rb - !3867
- [x] spec/serializers/upgrade_amendment_response_serializer_spec.rb - !3867
- [ ] spec/services/amendment_base_service_spec.rb - !4161
- [ ] spec/services/compliance_check_service_spec.rb - !4161
- [ ] spec/services/coupons/export_service_spec.rb - !4161
- [ ] spec/services/create_order_service_spec.rb - !4161
- [ ] spec/services/eoa_starter_bronze/calculate_next_price_service_spec.rb - !4161
- [ ] spec/services/eoa_starter_bronze/offer_renewals_count_service_spec.rb - !4161
- [ ] spec/services/find_or_create_customer_service_spec.rb - !4161
- [ ] spec/services/fix_cancelled_subscription_for_order_service_spec.rb - !4161
- [ ] spec/services/gitlab/admin/get_namespace_service_spec.rb - !4161
- [ ] spec/services/gitlab/base_trial_service_spec.rb - !4161
- [ ] spec/services/gitlab/hosted_plans/create_trial_service_spec.rb - !4161
- [ ] spec/services/gitlab/hosted_plans/extend_reactivate_trial_base_service_spec.rb - !4161
- [ ] spec/services/gitlab/hosted_plans/extend_trial_service_spec.rb - !4161
- [ ] spec/services/gitlab/hosted_plans/purchased_add_ons/ci_minutes_service_spec.rb - !4161
- [ ] spec/services/gitlab/hosted_plans/reactivate_trial_service_spec.rb - !4161
- [ ] spec/services/gitlab/namespaces/get_service_spec.rb - !4161
- [ ] spec/services/gitlab/namespaces/list_service_spec.rb - !4161
- [ ] spec/services/gitlab/namespaces/update_service_spec.rb - !4161
- [ ] spec/services/gitlab/self_hosted_plans/create_trial_service_spec.rb - !4161
- [ ] spec/services/gitlab/subscription/update_service_spec.rb - !4161
- [ ] spec/services/instance_review_create_service_spec.rb - !4161
- [ ] spec/services/license_notification_service_spec.rb - !4161
- [ ] spec/services/reconciliation_eligibility_service_spec.rb - !4161
- [ ] spec/services/reconciliation_service_spec.rb - !4161
- [ ] spec/services/reseller_notification_service_spec.rb - !4161
- [ ] spec/services/subscription_manager_service_spec.rb - !4161
- [ ] spec/services/update_contact_service_spec.rb - !4161
- [ ] spec/services/zuora_callbacks/credit_card_validation_service_spec.rb - !4161
- [x] spec/tasks/cleanup_customer_xss_payload_spec.rb - !4020
- [x] spec/viewers/transaction_viewer_spec.rb - !4020
- [x] spec/views/layouts/_impersonate_alert.html.haml_spec.rb - !4020
- [x] spec/workers/create_amendment_worker_spec.rb - !3866
- [x] spec/workers/salesforce/create_account_worker_spec.rb - !3866
- [x] spec/workers/salesforce/create_opportunity_worker_spec.rb - !3866
- [x] spec/workers/update_subscription_eoa_starter_bronze_offer_worker_spec.rb - !3866

Story points: 3.0