Make sure CI job `db:check-schema` can catch corrupted `structure.sql`

What does this MR do and why?

Make sure CI job db:check-schema can catch corrupted structure.sql.

• A merge request showing that it's not catching it before this merge request: !73300 (comment 717634556)

This is done by migrating to the previous major version in db:check-schema and then run migrations, it'll only be basing on db/structure.sql from last major version rather than blindly using the currently committed db/structure.sql.

See #343978 (closed) for details.

Side notes

• There's a huge disparity between current version and v13.12.9: https://gitlab.com/gitlab-org/gitlab/-/jobs/1726616416
  • Maybe just let it go...?

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.

Related to #343978 (closed)