groups/sig-cluster-lifecycle: update kubeadm-alerts group

- Allow anyone to join and post
- Have owners that can moderate

The idea is for only prow to post on this ML.
Once it sends its first message we will allow for future
messages too.

groups/sig-cluster-lifecycle/groups.yaml

@@ -18,7 +18,9 @@ groups:
 
     settings:
       WhoCanPostMessage: "ANYONE_CAN_POST"
-      ReconcileMembers: "true"
-    members:
+      WhoCanJoin: "ANYONE_CAN_JOIN"
+      MessageModerationLevel: "MODERATE_ALL_MESSAGES"
+      ReconcileMembers: "false"
+    owners:
       - [email protected]
       - [email protected]