Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -24,7 +24,7 @@ jobs:
         - ruby-head
         - jruby-head
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: