K3s module was moved from `nixos/.../k3s` to `nixos/.../rancher`.
`rke2/default.nix` was moved to `rancher/rke2.nix`, and some options from RKE2 were migrated into the common `default.nix` for backwards compatibility.
Manifest generation was also changed, instead of multi-doc YAML files the module now generates `kind: List` manifests.