Change GitHub CI to update checkout action to version 4

Author: piotrmurach

Diff for: .github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
       COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: