Revert "Update Node.js to v20 in GitHub Actions workflow"

This reverts commit 387411a.

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
         with:
-          node-version: '20'
+          node-version: '18'
       - name: Install modules
         run: npm install
       - name: Build the docker-compose stack