Revert "Update acm to v2.12"

This reverts commit 3010375.

This mistake is on me. I thought I checked that acm-2.12 was available
all the way back to 4.12 but it clearly it isn't.

Author: mbaldessari

values-hub.yaml

@@ -11,7 +11,8 @@ clusterGroup:
     acm:
       name: advanced-cluster-management
       namespace: open-cluster-management
-      channel: release-2.12
+      channel: release-2.11
+      #csv: advanced-cluster-management.v2.6.1
   projects:
     - hub
     - config-demo