Support ghc-9.12

[erikd]

Description

Should not be merged until SRP and most allow-newers are removed.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Self-reviewed the diff

[lehins]

We can definitely merge it with allow-newer stanzas, since the they are conditional on ghc-9.12

Good job on pushing the community to update their packages. Sorry you had to deal with difficult maintainers because of this!