Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -22,7 +22,7 @@ jobs:
       TEST_RAILS_VERSION: "${{ matrix.rails-version }}"
       CC_TEST_REPORTER_ID: "${{ secrets.CC_TEST_REPORTER_ID }}"
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: