docs: fix local contributor setup (#2206) - #4214

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@v3, ASzc/change-string-case-action@v6, docker/build-push-action@v5, docker/setup-buildx-action@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/