A few things on this:
• SUSE Rancher is supporting dockershim on 1.24+ for RKE1
• RKE1 is supported with no EOL set. RKE1 is currently the ONLY supported way to use docker with kubernetes
• If you want to use containerd then you'd need to switch away from RKE1 to something like RKE2 (the big selling point of RKE1 is being able to use docker)
• RKE1 and RKE2 are fundamentally different kubernetes distros. Much like something like EKS or kubeadm so there is no current direct migration path
• Since they are all kubernetes under the hood your deployment manifests should transfer between distros without any real friction.