Bump hard stop version, auto-detect min schema

What does this MR do and why?

We discovered that there are variables used by schema check before doing a migration which must be manually updated every time a migration squash happens.

This change automates detecting the minimum schema version acceptable to perform a migration to bring the database up to date. The minimum GitLab version is still hardcoded because this value should only be adjusted when an intentional hard stop is introduced.

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

Related to #370640 (closed)