Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ruby.yml

@@ -29,7 +29,7 @@ jobs:
         ruby-version: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', 'ruby-head']
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Ruby
     # To automatically get bug fixes and new Ruby versions for ruby/setup-ruby,
     # change this to (see https://github.com/ruby/setup-ruby#versioning):