CI: add GHC 9.6.1 (backport #8840)

[mergify]

This is an automatic backport of pull request #8840 done by Mergify.
Cherry-pick of ba407e0 has failed:

On branch mergify/bp/3.10/pr-8840
Your branch is up to date with 'origin/3.10'.

You are currently cherry-picking commit ba407e067.
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Changes to be committed:
	modified:   Cabal-tests/Cabal-tests.cabal
	modified:   cabal-install-solver/cabal-install-solver.cabal
	modified:   cabal-install/cabal-install.cabal
	modified:   cabal-testsuite/cabal-testsuite.cabal

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Cabal-tree-diff/Cabal-tree-diff.cabal

Cherry-pick of 561bf1b has failed:

On branch mergify/bp/3.10/pr-8840
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	both modified:   .github/workflows/validate.yml

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On branch mergify/bp/3.10/pr-8840
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On branch mergify/bp/3.10/pr-8840
Your branch is ahead of 'origin/3.10' by 8 commits.
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[ulysses4ever]

It looks like it may be easier to do it manually...

[Mikolaj]

Wow, how come. I specifically waited for this backport with some tweaks on 3.10 branch. To the point that I forgot what these tweaks were supposed to be. Probably some are already in this PR anyway. :)

[Mikolaj]

Almost all conflicts are in .github/workflows/validate.yml (the changed CI cache workaround) that should probably be just overwritten to the version from the PR.

[ulysses4ever]

@Mikolaj if you have energy to push it over the line, I'm all for it. I don't remember from top of the head if simply replacing the .yml is the right way: maybe 3.10 branch needs something more nuanced (e.g. my main worry is what version of cabal would be used and whether or not some version bumps should be done). But CI will show if this is acceptable.

[ulysses4ever]

Main work is in #8901.