Update checkout action to version 4

We were getting some warnings with version 3:

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: actions/checkout@v3. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

.github/workflows/tests.yml

@@ -35,7 +35,7 @@ jobs:
         ci_node_total: [5]
         ci_node_index: [0, 1, 2, 3, 4]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true