[Feature flag] Enable use_orders_api_preview_subscription

<!-- Title suggestion: [Feature flag] Enable description of feature -->

# Summary

<!-- Short description of what the feature is about and link to relevant other issues. -->

This issue is to rollout [Order preview for new subscriptions](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4476) on CustomersDot production,
that is currently behind the [`use_orders_api_preview_subscription`](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags/150/edit) feature flag.

## Owners

- Team: Fulfillment Platform
- Most appropriate Slack channel to reach out to: `#g_fulfillment_platform`
- Best individual to reach out to: @aish.sub 
- PM: @tgolubeva 

## Stakeholders

<!--
Are there any other stages or teams involved that need to be kept in the loop?

- GITLAB_HANDLE of a PM
- The Support Team
- The Delivery Team
-->

## The Rollout Plan

- Partial Rollout with specific Customers on CustomersDot, [via Unleash](https://docs.gitlab.com/ee/operations/feature_flags.html#user-ids)
- Rollout on CustomersDot for a certain period (How long)
- Percentage Rollout on CustomersDot
- Rollout Feature for everyone as soon as it's ready

<!-- Which dashboards from https://dashboards.gitlab.net are most relevant? Sentry errors reports can also be useful to review -->

## Testing User/Customer IDs

<!--
List any Customer IDs for Customers that are happy to have this feature turned on early.

Remove if not applicable
-->

## Expectations

### What are we expecting to happen?

<!-- Describe the expected outcome when rolling out this feature -->

When customer's preview subscription before purchase, the request in the backend uses Zuora's [Preview an Order API](https://www.zuora.com/developer/api-reference/#operation/POST_PreviewOrder) instead of Subscribe API.

### What might happen if this goes wrong?

<!-- Should the feature flag be turned off? Any MRs that need to be rolled back? Communication that needs to happen? What are some things you can think of that could go wrong - data loss or broken pages? -->

Customer's may not be able to preview subscription accurately before purchase.

### What can we monitor to detect problems with this?

<!-- Which dashboards from https://dashboards.gitlab.net are most relevant? -->

## Rollout Steps

### Rollout on non-production environments

- [x] Ensure that the feature MRs have been deployed to non-production environments, i.e. `customers-staging`.
- [x] Consider announcing that the flag will be enabled to potentially impacted groups, e.g. `#s_fulfillment_engineering`
- [x] Enable the feature globally on non-production environments by applying the appropriate environment, e.g. `customers-staging` and `customers-development`.
- [x] Notify the Software Engineers in Test (SETs) for Fulfillment via Slack and in a comment in this issue when enabling a feature flag in `customers-staging` as it can impact the QA pipeline.
- [x] Verify that the feature works as expected. Posting the QA result in this issue is preferable.

### Preparation before production rollout

- [x] Ensure that the feature MRs have been deployed to production
- [ ] Check if the feature flag change needs to be accompanied with a
  [change management issue](https://about.gitlab.com/handbook/engineering/infrastructure/change-management/#feature-flags-and-the-change-management-process).
  Cross link the issue here if it does.
- [x] Ensure that you or a representative in Fulfillment can be available for at least 2 hours after feature flag updates in production.
  If a different developer will be covering, or an exception is needed, please inform the Fulfillment team in #s_fulfillment_engineering
- [ ] Ensure any documentation has been updated
- [ ] Announce on [the feature issue](ISSUE LINK) an estimated time this will be enabled on production if applicable
- [ ] If the feature might impact the user experience, notify `#s_fulfillment` and potentially `#support_gitlab-com`, as well as your team channel ([more guidance when this is necessary in the dev docs](https://docs.gitlab.com/ee/development/feature_flags/controls.html#communicate-the-change)).

### Global rollout on CustomersDot production

- [x] Confirm the feature flag is enabled on `staging` without incident
- [ ] Roll out the feature to targeted testing users first (if applicable)
- [x] If applicable, [incrementally roll out](https://docs.gitlab.com/ee/operations/feature_flags.html#percent-rollout) the feature.
- [x] Verify the change has the desired outcome with the limited rollout before enabling the feature globally on production.
- [x] Enable the feature globally on production environment
- [x] Announce on [the feature issue](ISSUE LINK) that the feature has been globally enabled.
- [ ] Wait for [at least one day for the verification term](https://about.gitlab.com/handbook/product-development-flow/feature-flag-lifecycle/#including-a-feature-behind-feature-flag-in-the-final-release).

### Release the feature

After the feature has been [deemed stable](https://about.gitlab.com/handbook/product-development-flow/feature-flag-lifecycle/#including-a-feature-behind-feature-flag-in-the-final-release),
the clean up should be done as soon as possible to permanently enable the feature and reduce complexity in the codebase.

<!-- The checklist here is to help stakeholders keep track of the feature flag status -->
- [x] Create a merge request to remove `<feature-flag-name>` feature flag
  - [x] Remove all references to the feature flag from the codebase
- [ ] Ensure that the cleanup MR has been deployed to both staging and production
- [ ] Disable the feature flag via the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
- [ ] Ensure production environment is still working as expected
- [ ] Remove the Unleash feature flag from the [CustomersDot dashboard](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags)
- [ ] Close this rollout issue.

## Rollback Steps

- [ ] This feature can be disabled by performing the following actions:
  - [ ] Disable the Unleash flag on the [feature flag page](LINK TO THE FEATURE FLAG EDIT PAGE)

Story points: 2.0