Update actions/checkout to v4

This should quiet down the "Annotations" in workflow runs.

See: https://github.com/DataWraith/ascii85gem/actions/runs/9014724946

> 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/ruby.yml

@@ -32,7 +32,7 @@ jobs:
           - 'truffleruby'
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: