xmonad.cabal: update constraints on 'base'

- Our code does not compile with versions prior to 4.6, because we need
   System.Environment.lookupEnv.

 - Our code does not compile with version 4.12 (GHC 8.6.x) and beyond.

Closes https://github.com/xmonad/xmonad/issues/180.

xmonad.cabal

@@ -62,7 +62,7 @@ library
                         XMonad.StackSet
     other-modules:      Paths_xmonad
 
-    build-depends: base < 5 && >=3,
+    build-depends: base >= 4.6 && < 4.12,
                    containers,
                    data-default,
                    directory,