cabal: Bump version to 0.18.1

Bump version to 0.18.0

cabal: Update tested GHC versions

This syncs the cabal file with the reality of our CI.

INSTALL: Suggest newer tagged releases

xmonad.cabal: Specify default-language

Required for cabal-version >= 1.10

xmonad version 0.15

xmonad version 0.14.2

xmonad version 0.14.1

xmonad version 0.14

Release 0.13