Upgrade Node.js to version 16 in the CI workflow

Version 14 that we used before is now in maintenance mode, so we should
upgrade to the most recent LTS version.

Moreover, use the most recent `setup-node` workflow version and syntax;
see https://github.com/actions/setup-node#usage.

.github/workflows/ci.yml

@@ -9,10 +9,10 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Use Node.js 14 LTS
-        uses: actions/setup-node@v1
+      - name: Use Node.js 16 LTS
+        uses: actions/setup-node@v2
         with:
-          node-version: 14.x
+          node-version: '16'
 
       - name: Install Gulp
         run: npm install -g gulp-cli