Rename ghc@9 to ghc and update to 9.4.1

[triallax]

• [x] Have you followed the guidelines for contributing?
• [x] Have you ensured that your commits follow the commit style guide?
• [x] Have you checked that there aren't other open pull requests for the same formula update/change?
• [ ] Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [ ] Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• [x] Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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See https://github.com/Homebrew/homebrew-core/pull/106859#issuecomment-1200378931. I have no idea if I did this the right way, reviews are welcome.

Note that GHC 9.4.x will be the last series to support a Make-based build system. From GHC 9.6 onward, only a build system called Hadrian will be supported. See https://www.haskell.org/ghc/blog/20220805-make-to-hadrian.html for details.