Remove support for GHC < 9.0 (#119)

* Update lower version bounds to GHC 9.0

Update base from >=4.7 to >=4.15 and ghc from >=8.4 to >=9.0 to match
the Hackage revision and CI build matrix, which don't support GHC <9.0.

Fixes #128

* Remove support for GHC < 9.0

* Remove conditional source code for ghc < 9.0

Author: blackgnezdo

ChangeLog.md

@@ -4,6 +4,7 @@
 # 0.4.7.0
 - Support GHC 9.12.
 - Support GHC 9.14.
+- Remove support for GHC < 9.0.
 
 # 0.4.6.0
 - Support GHC 9.10.