[tacker] 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: I21fc820c305389ff88e180c4aa36459c0fe652fe

deliverables/rocky/tacker-horizon.yaml

@@ -11,5 +11,9 @@ releases:
       - hash: b701cb4d494cd05a409915aa7a455670f1b30dc6
         repo: openstack/tacker-horizon
     version: 0.12.0
+  - version: rocky-em
+    projects:
+      - repo: openstack/tacker-horizon
+        hash: b701cb4d494cd05a409915aa7a455670f1b30dc6
 repository-settings:
   openstack/tacker-horizon: {}