Drop support for GHC 9.0 early

[michaelpj]

GHC 9.0 is still in our support window, but:

1. Anecdotally, it's an unpopular release, with most people having gone straight from 8.10 to 9.2
2. It is relatively expensive for us to support, since the exactprint in-tree migration happened in 9.2. So if we drop it we can get rid of a bunch of CPP.

For these reasons I propose dropping GHC 9.0 after the next release.

[michaelpj]

cc @hasufell

[hasufell]

+1