CI needs updating to more recent GHC version

[JacquesCarette]

Looks like the CI uses GHC 8.10.7 (see .github/workflows/ci-ubuntu.yml) and cabal 3.6.2.0. Agda itself now seems to use stack and GHC 9.8.1. That's a very large gap!

[jamesmckinna]

And now... more upgrading required:

Warning: As of 2023-09-09, haskell/action/setup is no longer maintained, please switch to haskell-actions/setup (note: dash for slash).
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**              This action is DEPRECATED.                               **
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**              Please use haskell-actions/setup instead.                **
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**              (Note the dash instead of the slash.)                    **
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