Drop support for GHCs older than 8.4

[clyring]

GHC 8.4 was first released in March 2018, very nearly six years ago. I think that's certainly an adequate support window. I propose making our next release (0.12.1.0) the last one to support any GHC version older than this.

There's no pressing need to do so--I'm just tired of getting paper cuts from things like the stuff in GHC.Magic not being re-exported in base-4.9 or Prelude not re-exporting (<>) until base-4.11 or COMPLETE pragmas not existing with ghc-8.0. (We could probably even drop a few more old versions than this, but I don't remember running into anything inconvenient about 8.4 or 8.6 in bytestring. MonadFail changes are annoying but we aren't affected.)

[Bodigrim]

I'm very happy to drop GHC < 8.6 immediately, even in 0.12.1.0.

[clyring]

done in #682