CI: Build with ghc 9.12 and update cache action

Also drop building with GHC versions lower than `9.6`.

Author: erikd

.github/workflows/haskell.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ghc: [ "8.10.7", "9.2.7", "9.6.6", "9.8.2", "9.10.1" ]
+        ghc: [ "9.6", "9.8", "9.10", "9.12" ]
         os: [ubuntu-latest, macos-latest, windows-latest]
 
     steps:
@@ -51,7 +51,7 @@ jobs:
     - name: Set cache version
       run: echo "CACHE_VERSION=9w76Z3Q" >> $GITHUB_ENV
 
-    - uses: actions/cache@v2
+    - uses: actions/cache@v4
       name: Cache cabal store
       with:
         path: ${{ steps.setup-haskell.outputs.cabal-store }}