gitlab-rails: support skipping post-migrations in db checks

What does this MR do?

Backport !1548 (merged) to 16.5 as the current supported release for GitLab Dedicated.

Merge branch 'rails-deps-skip-post' into 'master'

gitlab-rails: support skipping post-migrations in db checks

See merge request !1548 (merged)

(cherry picked from commit de0b3e80)

bf41476c gitlab-rails: support skipping post-migrations in db checks

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io