CI: bump action/checkout from v4 to v6

Author: orien

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
             experimental: true
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby_version }}