Update Node.js version to 20

Bump Node.js version from `18` to `20` in deploy and test-deploy GitHub Actions workflows to ensure compatibility with the latest Node.js features and security updates. This is necessary because Docusaurus 3.9 drops support for Node.js 18: https://docusaurus.io/blog/releases/3.9#dropping-nodejs-18

Author: josh-wong

.github/workflows/deploy.yml

@@ -18,7 +18,7 @@ jobs:
           fetch-depth: 0
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
           cache: npm
 
       - name: Install dependencies

.github/workflows/test-deploy.yml

@@ -18,7 +18,7 @@ jobs:
           fetch-depth: 0
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
           cache: npm
 
       - name: Install dependencies