fix(ci): set node version to 20

Author: tengomucho

.github/workflows/doc-build.yml

@@ -27,7 +27,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
           cache-dependency-path: "kit/package-lock.json"
 
       - name: Set environment variables