Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/haskell.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: actions/setup-haskell@v1
       with:
         ghc-version: '8.6.5'