feat(cli): remove the CLI plugin extension mechanism - #3702

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11, bufbuild/buf-action@5150a1eef5c10b6a5cf8a69fc872f24a09473195. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/