users-guide: Rename 8.12 to 9.0
GHC 8.12.1 has been renamed to GHC 9.0.1.
See also: https://mail.haskell.org/pipermail/ghc-devs/2020-July/019083.html
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GHC 8.12.1 has been renamed to GHC 9.0.1.
See also: https://mail.haskell.org/pipermail/ghc-devs/2020-July/019083.html
[skip ci]