ci: remove duplicate top-level keys in CI workflow; matrix Node versions

Author: luisfer

.github/workflows/ci.yml

@@ -1,30 +1,5 @@
 name: CI
 
-on:
-  push:
-    branches: [ main ]
-  pull_request:
-    branches: [ main ]
-
-jobs:
-  build-test:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-      - name: Setup Node
-        uses: actions/setup-node@v4
-        with:
-          node-version: '20'
-          cache: 'npm'
-      - name: Install
-        run: npm ci
-      - name: Build
-        run: npm run build
-      - name: Test
-        run: npm test -- --ci
-name: CI
-
 on:
   push:
     branches: [ main ]