Raise minimum GHC version to 8.6
Raise minimum GHC version to 8.6

commit 795986aaf33e ("Remove unneeded CPP now that GHC 8.6 is the minimum")
broke the 8.4 build.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc