Document which GHC version introduced GHC2021

https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/control.html doesn't tell since which GHC2021 is available.

I actually have no idea what is the best way (without having all GHC versions installed locally to try) to find out which is the first GHC version supporting GHC2021. Looking through the all the manual going back is not great.

Compare to e.g. https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/numeric_underscores.html#extension-NumericUnderscores, it tells immediately that NumericUnderscores is there since GHC-8.6.1

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