Fix some warnings when bootstrapping with GHC 9.0

This fixes two classes of warnings that appear when bootstrapping with GHC 9.0:

* `ghc-boot.cabal` was using `cabal-version: >=1.22`, which `cabal-install-3.4`
  now warns about, instead recommending the use of `cabal-version: 1.22`.
* Several pattern matches were producing `Pattern match(es) are non-exhaustive`
  because of incorrect CPP. The pattern-match coverage checker _did_ become
  smarter in GHC 9.1, however, so I ended up needing to keep the CPP, adjusting
  them to use `#if __GLASGOW_HASKELL__ < 901` instead.