Update actions/checkout that warns

"Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/."

https://github.com/rails/activeresource/actions/runs/7646036452

.github/workflows/ci.yml

@@ -80,7 +80,7 @@ jobs:
               BRANCH: main
             experimental: true
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         env: ${{ matrix.env }}