[horizon] Transition Rocky to EM

This transition the rocky 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.

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.

Change-Id: Ibca76b3d3209fbb79674ee8b900cd4554b88c27e

deliverables/rocky/horizon.yaml

@@ -49,6 +49,10 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: ed23eb60d4674e7d50f9f13cc926d3e3bb4f1121
+  - version: rocky-em
+    projects:
+      - repo: openstack/horizon
+        hash: ed23eb60d4674e7d50f9f13cc926d3e3bb4f1121
 repository-settings:
   openstack/horizon: {}
 branches: