Use actions/checkout v6

Author: rgrove

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ jobs:
           - truffleruby-head
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true