Update actions/checkout version to v6

Author: bernd

.github/workflows/ruby.yml

@@ -16,7 +16,9 @@ jobs:
         ruby: ["2.7", "3.0", "3.1", "3.2", "3.3", "3.4"]
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
+      with:
+        persist-credentials: false
 
     - name: Set up Ruby ${{ matrix.ruby }}
       uses: ruby/setup-ruby@v1