Revert "Revert "Re-enable lts based testing""

This reverts commit 8a8d5f71b1.

There will be a separate GH Actions workflow for rebuilding the manpage,
and generatemanpage will be dropped from xmonad.cabal (see
https://github.com/xmonad/xmonad/issues/283) therefore the revert no
longer makes sense and the stack workflow can indeed be reverted back to
a working state so we can continue from there.

For more information, see the following revenge revert storm:

Related: https://github.com/xmonad/xmonad/issues/283
Related: https://github.com/xmonad/xmonad/pull/260
Related: https://github.com/xmonad/xmonad/pull/261
Related: https://github.com/xmonad/xmonad/pull/265
Related: https://github.com/xmonad/xmonad/pull/266
Related: https://github.com/xmonad/xmonad/pull/267
Related: https://github.com/xmonad/xmonad/pull/268

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        resolver: [nightly]
+        resolver: [lts-16, lts-15, lts-14, lts-12]
 
     steps:
       - name: Clone project

stack.yaml

@@ -1,19 +1,10 @@
-resolver: nightly-2021-01-05
-
-flags:
-  xmonad:
-    generatemanpage: true
+resolver: lts-16.22
 
 packages:
 - ./
 
 extra-deps:
 - X11-1.9.2
-- pandoc-2.11.3.2
-- citeproc-0.3.0.3
-- commonmark-0.1.1.2
-- commonmark-extensions-0.2.0.4
-- commonmark-pandoc-0.2.0.1
 
 nix:
   packages: