Update setup-node action to version 4

We were getting a warning with version 3:

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: actions/setup-node@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

@@ -40,7 +40,7 @@ jobs:
         with:
           bundler-cache: true
       - name: Setup NPM
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           cache: "npm"
           node-version-file: ".node-version"