[horizon] 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: I4a3b4c5051cd888d824c64c9f93a6bb4e6bd10e5

deliverables/xena/horizon.yaml

@@ -34,6 +34,10 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: 07343c853af4e9b74fe11faaa2bbc1a8c1e62b78
+  - version: xena-em
+    projects:
+      - repo: openstack/horizon
+        hash: 07343c853af4e9b74fe11faaa2bbc1a8c1e62b78
 branches:
   - name: stable/xena
     location: 20.1.0