ci: update actions/checkout action to v6

Author: Watson1978

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
@@ -44,7 +44,7 @@ jobs:
 
     name: ${{ matrix.gemfile }} - Ruby ${{ matrix.ruby-version }} on ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: