docs: 죽은 umbrella 절대 URL 제거 (agent-plugin#221 Opt 2) - #87

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: docker/build-push-action@v6, docker/setup-buildx-action@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/