[FF cleanup] Auto renewal, Reconciliation Preview and Order Preview GraphQL endpoint

### Problem

The scope of this issue is to perform cleanup tasks for  Auto renewal, Reconciliation Preview and Order Preview GraphQL endpoint feature flags noted below:

- [x] `use_orders_api_to_preview_quoted_invoice_amount` ([link to original rollout issue](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5068)) - [Cleanup MR](https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/6254)
- [x] `use_orders_api_preview_in_upcoming_saas_mailer` ([link to original rollout issue](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5070)) - [Cleanup MR](https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/6266)
- [x] `use_orders_api_in_preview_graphql_resolver ` ([link to original rollout issue](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5072)) - [Cleanup MR](https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/6278)

### Proposal

Perform `Release the feature` tasks:

1. **FF `use_orders_api_to_preview_quoted_invoice_amount`**

* [x] Create a merge request to remove `<feature-flag-name>` feature flag
  * [x] Remove all references to the feature flag from the codebase
* [x] Ensure that the cleanup MR has been deployed to both staging and production
* [x] Disable the feature flag via the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
* [x] Ensure production environment is still working as expected
* [x] Remove the Unleash feature flag from the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
* [ ] Close this issue.

2. **FF `use_orders_api_preview_in_upcoming_saas_mailer`**

* [x] Create a merge request to remove `<feature-flag-name>` feature flag
  * [x] Remove all references to the feature flag from the codebase
* [x] Ensure that the cleanup MR has been deployed to both staging and production
* [x] Disable the feature flag via the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
* [x] Ensure production environment is still working as expected
* [x] Remove the Unleash feature flag from the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
* [ ] Close this issue.

3. **FF `use_orders_api_in_preview_graphql_resolver`**

* [x] Create a merge request to remove `<feature-flag-name>` feature flag
  * [x] Remove all references to the feature flag from the codebase
* [x] Ensure that the cleanup MR has been deployed to both staging and production
* [x] Disable the feature flag via the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
* [x] Ensure production environment is still working as expected ([related log](https://console.cloud.google.com/logs/query;cursorTimestamp=2023-01-12T18:37:45.384Z;query=%22OrderPreview%22%0Atimestamp%3D%222023-01-12T18:37:45.384Z%22%0AinsertId%3D%22wha5a5fctxmgw%22?project=gitlab-subscriptions-prod))
* [x] Remove the Unleash feature flag from the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
* [ ] Close this issue.

Story points: 3.0