[monasca] Transition Xena to EM

This transition the stable/xena branch to extended maintenance.
Changes for bugfixes and things the team deems important are
still encouraged, but there will no longer be official releases
off of the branch after the transition.

Please +1 if the team is ready for us to proceed with this
transition, or -1 if there are any final backports currently in
flight that we should wait for. For the latter case, please
update the patch with the new commit hash after doing a final
release to get those changes out so we know to proceed with the
transition.

The transition deadline is April 20th, 2023.

Change-Id: Idc3aa79d51a4291c25b61f8f3e8d79597f97b0ad

deliverables/xena/monasca-notification.yaml

@@ -15,6 +15,10 @@ releases:
       - repo: openstack/monasca-notification
         hash: fa2454a3c9c524773891e53d2d61f9227a9a2d46
     diff-start: 4.0.0
+  - version: xena-em
+    projects:
+      - repo: openstack/monasca-notification
+        hash: fa2454a3c9c524773891e53d2d61f9227a9a2d46
 branches:
   - name: stable/xena
     location: 5.0.0.0rc1