Update checkout action in GitHub Actions

Author: benedikt

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
           - 3.2
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}