Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
         ruby: [3.3]
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up Ruby