Update actions/checkout action to v6 (#4)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

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