Decouple migrations: Re-introduce schema backwards compatibility check in CI

Created by: efritz

Partially implements Step 0 in RFC 469.

Mechanically check backwards compatibility of migrations via a check in our buildkite pipeline that should run the unit tests of the most recently tagged minor version (with a patch value of zero) against the schema in the current commit.

We should likely put in a few escape hatches as well so that we can explicitly ignore issues that might arise around:

• Databases powering experimental features (e.g., codeinsights can make breaking changes with less of a blast radius)
• Flaky unit tests from previous releases

It is likely sufficient to simply have a list of test names/patterns to ignore in this check.

This check should only be re-introduced after the announcement made in #25251 (closed).