Fix broken links to the React 19 Upgrade Guide - #8601

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-node@v4, actions/upload-artifact@v4, dawidd6/action-download-artifact@268677152d06ba59fcec7a7f0b5d961b6ccd7e1e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/