Clusterclaims-controller: managedcluster deletion keyed solely on clusterclaim.spec.namespace with no local ownership check

A flaw was found in the clusterclaims-controller component of multicluster engine (MCE). A tenant with standard permissions to create and delete ClusterClaim resources can exploit this by manipulating the `spec.namespace` field. This allows the tenant to specify and delete any ManagedCluster, including the hub's local-cluster or other tenants' clusters, due to a missing ownership check. This vulnerability can lead to a denial of service by enabling unauthorized deletion of ManagedClusters.