add warnings when config cluster-namespace as karmada-system

Signed-off-by: Poor12 <shentiecheng@huawei.com>

pkg/karmadactl/join.go

@@ -125,6 +125,9 @@ func (j *CommandJoinOption) Validate() error {
 		return fmt.Errorf("invalid cluster name(%s): %s", j.ClusterName, strings.Join(errMsgs, ";"))
 	}
 
+	if j.ClusterNamespace == util.NamespaceKarmadaSystem {
+		klog.Warningf("karmada-system is always reserved for Karmada control plane. We do not recommend using karmada-system to store secrets of member clusters. It may cause mistaken cleanup of resources.")
+	}
 	return nil
 }