Upgrade Node.js to version 20 in CI workflow

Author: kiuma

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x]
 
     steps:
       - name: Checkout code

.github/workflows/npm-publish.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
           registry-url: 'https://registry.npmjs.org'
 
       - name: Cache node modules