Add option to enable/disable review-apps on stable branch

What does this MR do and why?

Describe in detail what merge request is being backported and why

While investigating Failure in `gitlab-rails-ee` job in review-app'... (#429711 - closed), we were wondering if the stable branch should also adopt the enable/disable review app pipeline config with the CI_REVIEW_APPS_ENABLED flag.

Add a configuration to disable/enable review apps in stable branch. Backport the CI config added in Re-enable review-apps + CI/CD feature flag (!130644 - merged).

(cherry picked from commit af23e32e)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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