Prevent adding schema deltas to old `SCHEMA_VERSION`s

[richvdh]

Once an installation's database has been updated to a given SCHEMA_VERSION, any delta files which appear in older versions are ignored.

This means we have a ticking timebomb/footgun in which we accidentally merge a PR which adds a delta file to an old SCHEMA_VERSION, which is then ignored on existing installations.

Ideally we want to make sure that, once develop is updated to a given SCHEMA_VERSION, no delta files are added to older versions.

[reivilibre]

What woud you suggest? A CI check for that? (main issue is that you can merge a branch on a stale CI check: if my CI passes and you later change develop's SCHEMA_VERSION, nothing will alert me to that or stop me merging.)

[richvdh]

well yes, therein lies the problem - we'd have to do some research.

One option would be to enable this checky-box, but I think it would rapidly get painful, given we tend to have multiple PRs in flight at any given time:

So maybe we have to actually run a CI job on develop itself, which diffs against the commit where SCHEMA_VERSION was last bumped and checks that nothing has been added to older versions (and files an issue if it has)?

[clokep]

I think @richvdh marked this as fixed by #13063.

[DMRobertson]

This as followed-up by #15466.