Update actions/checkout from v4 to v6

Author: fkmy

.github/workflows/test.yml

@@ -61,7 +61,7 @@ jobs:
             ruby-version: 2.2
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Ruby ${{ matrix.ruby-version }}
         uses: ruby/setup-ruby@v1
         with: