Update CI for newer cabal, use only GHC 9.12 on windows consistently

[carbolymer]

Changelog

- description: |
    Update CI for newer cabal, use only GHC 9.12 on windows consistently
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...
# uncomment at least one main project this PR is associated with
  projects:
   - cardano-api
  # - cardano-api-gen
   - cardano-rpc
  # - cardano-wasm

Context

Since we switched to GHC 9.12 for windows builds, we may switch to GHC 9.12 everywhere.
This PR also adds GHC 9.6.7 (linux only) to nix, to check that we're still building fine and we're able to integrate in cardano-node.
Unused GHC versions are removed from windows build GHA.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff