docs: add compatibility matrix to README - #208

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, docker/build-push-action@ca052bb54ab0790a636c9b5f226502c73d547a25, docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f, docker/setup-qemu-action@c7c53464625b32c7a7e944ae62b3e17d2b600130. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/