Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/haskell.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
     - uses: actions/setup-haskell@v1
       with:
         ghc-version: '8.10'