Remove CPP for no longer supported GHC <9.0

[jhrcek]

The package no longer builds with GHCs older than 9.0.

When you try cabal build with ghc 8.10.7, you get this error:

Failed to build hie-compat-0.3.1.2.
Build log (
/home/jhrcek/.cabal/logs/ghc-8.10.7/hie-compat-0.3.1.2-6cd25c57f80a4565be3fe6faa473ae59b8a96c9dd34aa406aef36d991f3ccd54.log
):
Configuring library for hie-compat-0.3.1.2..
Preprocessing library for hie-compat-0.3.1.2..
Error: cabal-3.10.2.1: can't find source for Compat/HieAst in .,
dist/build/autogen, dist/build/global-autogen

Error: cabal: Failed to build hie-compat-0.3.1.2 (which is required by
test:hiedb-tests from hiedb-0.6.0.0). See the build log above for details.

Support for ghc 8.10 and older was removed from hie-compat in haskell/haskell-language-server#3434

So I'd say that we can safely remove CPP related to older GHCs from this repo too AND mention it in the changelog.

[jhrcek]

This is the first release following drop of support from hie compat several months before this was released, so adding it here retrospectively.