Re-enable review-apps + CI/CD feature flag

Context

Follow-up of !130528 (merged).

What does this MR do and why?

• Re-enable review-apps (The root cause issue was fixed)
• Introduce a CI/CD environment variable to be able to quickly enable/disable further review-apps deployments.

Proof of work

CI_REVIEW_APPS_ENABLED = "true2"

✅ The pipeline https://gitlab.com/gitlab-org/gitlab/-/pipelines/987966852 doesn't contain the start-review-app-pipeline job:

CI_REVIEW_APPS_ENABLED = "true"

✅ The pipeline https://gitlab.com/gitlab-org/gitlab/-/pipelines/987966852 contains the start-review-app-pipeline job:

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.