Overhaul CI

[hasufell]

What this PR does:

• test the sdist, not the project!
  • we want to know it can be built from hackage
  • that means we also get rid of cabal freeze
  • and inject --ghc-options='-Werror' manually
• automatically test all GHCs in the tested-with field
  • we use https://github.com/Kleidukos/get-tested
  • due to an outstanding issue we need a minor workaround: Multiple runner versions? Kleidukos/get-tested#93
• upload the plan.json files
• automatically test the last 4 LTS stackage releases
  • this code is custom shell, but very simple
  • this should also implicitly test that we're compatible with the shipped boot libraries
• test haddock
• run cabal check
• remove caching, we don't really need it
• use haskell/ghcup-setup over haskell-actions/setup
  • it's simpler and more predictable
  • allows better advanced configuration
• add matrix to mhs job just in case
• update actions/checkout to v6

[sjshuck]

Reviewing this now @hasufell, thanks.

due to an outstanding issue we need a minor workaround

Is this a blocker?

[hasufell]

No, the workaround is in the workflow file. I documented it there.

[sjshuck]

Double parens needed?

[hasufell]

I'm not sure I follow. The whole thing is already in single parens.

[hasufell]

Cheers