Upgrade Node.js to version 18 in the CI workflow

Version 16 that we used before is now in maintenance mode, so we should
upgrade to the most recent LTS version. For more information on the
Node.js release schedule please refer to
https://github.com/nodejs/release#release-schedule.

.github/workflows/ci.yml

@@ -12,10 +12,10 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v3
 
-      - name: Use Node.js 16 LTS
+      - name: Use Node.js 18 LTS
         uses: actions/setup-node@v3
         with:
-          node-version: '16'
+          node-version: '18'
 
       - name: Install Gulp
         run: npm install -g gulp-cli