Update actions/checkout in GitHub CI

Author: collimarco

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
         ruby-version: ['3.0', '3.1', '3.2', '3.3', '3.4']
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: