[horizon] Transition Stein to EM

This transition the stein 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: Ib677b840a83e7770a92d606cd77583d4d80e1b2a

deliverables/stein/horizon.yaml

@@ -49,6 +49,10 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: 9e0e333ab5277b6c396f602862ff90398cb0242b
+  - version: stein-em
+    projects:
+      - repo: openstack/horizon
+        hash: 9e0e333ab5277b6c396f602862ff90398cb0242b
 repository-settings:
   openstack/horizon: {}
 branches: