Remove Riza sponsor image from README and docs - #4581

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: bufbuild/buf-lint-action@v1, bufbuild/buf-setup-action@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/