Identify undeployed commits and rollbacks in security merge requests

What does this MR do?

This MR prevents merging security fixes on master that does not start from the auto_deploy candidate commit from pages.

Example MRs from security:

• From master (unreleased commits) https://gitlab.com/gitlab-org/security/gitlab-pages/-/merge_requests/60#note_1245917598
• From currently deployed SHA https://gitlab.com/gitlab-org/security/gitlab-pages/-/merge_requests/61#note_1245951860
• From an old master commit (rollback) https://gitlab.com/gitlab-org/security/gitlab-pages/-/merge_requests/62#note_1245961397

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines