Rancher using runc instead of Docker?

AFAIU Docker will be (or has been) ditched for Kubernetes, so I wonder how I can install Kubernetes Cluster using runc via Rancher?

It’s a little more complicated than that. dockershim has been deprecated. It’s only one part of the overall Docker ecosystem. However, Mirantis picked up support for it, so it will be possible to install it and continue to use Docker.

Your statement can perhaps be rephrased as, “We don’t need all of Docker installed just to have a runtime engine,” and this is correct. Rancher currently deploys RKE, which requires Docker. We have released RKE2, which operates similar to K3s as a single binary that bundles containerd as the CRI. This will replace RKE in the future, long before the dockershim drops out of Kubernetes.