Ensure we're caught up with DB migrations squashing

Context

In Investigate db:check-schema slowdown (gitlab-org/quality/engineering-productivity/team#564 - closed), we discovered that DB migrations not being squashed is causing slowdowns in DB-related CI/CD jobs.

It also impacts the duration of background migration specs substantially (more info in Optimize background migrations specs (#519578 - closed)).

Goals

• Squash DB migrations until at least 17-0-stable-ee
• We have a process to do those DB squashes regularly (see gitlab-org/quality/engineering-productivity/team#571 (closed))