drop support for unsupported GHC versions

## Summary

We are building, testing and CI'ing with releases back to GHC 8.10.  The lowest
currently recommended version of GHC is 9.6.x.  See:

- https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC%20Status#most-recent-ghc-major-versions
- https://endoflife.date/ghc

We should consider dropping support for GHC < 9.6.