Avoids GitHub Actions deprecations and uses newer Node.js runtime.

https://github.com/actions/checkout/releases/tag/v4.0.0
https://github.com/docker/metadata-action/releases/tag/v5.0.0
https://github.com/docker/setup-qemu-action/releases/tag/v3.0.0
https://github.com/docker/setup-buildx-action/releases/tag/v3.0.0
https://github.com/docker/login-action/releases/tag/v3.0.0
https://github.com/docker/build-push-action/releases/tag/v5.0.0