Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
 
       - name: Configure Git