haddock: Remove compatibility shims for GHC < 8.4 from haddock-library

Removes old compatibility shims from haddock-library GHC versions < 8.4. This allows to remove the CompatPrelude module and all occurrences of CPP in the library.

I removed some of the old GHC compilers from the tested-with clause, but it is still kind of a lie since we currently don't have a CI matrix for these versions. The only lower bound that I bumped up is for base, I didn't check whether the other lower bounds are still building.