Tooling to facilitate postgres upgrades

[mzabani]

If one updates postgres, they currently need to coordinate a PR to upgrade the expected schema.

It will be simpler if we put one folder per major PG version in the expected-schema folder, and at runtime decide which to use.

Users would still need to ensure they update both if they keep multiple versions of postgres in different environments, but that seems inevitable.

However, for an upgrade in all environments, this will make the requirement of coordination simpler, allowing a PR that includes the expected schema of the new version to be merged a bit earlier.

It will also allow reverting the DB more easily if something goes wrong.

This is quite similar to what @TravisCardwell did in https://www.extrema.is/blog/2023/03/28/codd-experiment-2-postgresql-upgrade, IIUC, but would be builtin to codd.