Update actions/checkout action to v4

Author: renovate[bot]

lib/molinillo/.github/workflows/crystal.yml

@@ -11,7 +11,7 @@ jobs:
       image: crystallang/crystal
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: shards install
     - name: Run tests