feat: automatically fix 1.20 items when switching to 1.21 - #1226

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-java@v4, gradle/actions/setup-gradle@v4, nick-fields/retry@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/