Privilege escalation to cluster-admin on the Rancher local cluster through Fleet's agent management controller

SUSE Rancher Fleet clusters reference a kubeconfig secret that the controller uses to install and manage a Fleet agent on the target cluster. The controller did not sufficiently restrict which kubeconfig secrets a Fleet Cluster object was allowed to reference. As a result, a Fleet cluster defined in a namespace an ordinary user controls could be associated with administrative credentials for the management cluster itself, causing the controller to manage the management cluster on that user's behalf and to run that user's Fleet content there with elevated privileges. This affects SUSE Rancher Fleet 0.16.0 up to 0.16.3, 0.15.0 up to 0.15.8, 0.14.0 up to 0.14.12, 0.13.0 up to 0.13.17 and possible older versions.