CI: Use newest actions/checkout

GHA warns us that

> Node.js 16 actions are deprecated. Please update the following actions to
> use Node.js 20: actions/checkout@v2. [...]

.github/workflows/tests.yml

@@ -28,7 +28,7 @@ jobs:
           - "faraday-2"
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1