[horizon] Transition Wallaby to EM

This transition the wallaby 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.

The transition deadline is November 2nd, 2022.

Change-Id: I930214ed8c119bb62b3d06fab34eb19e3c5e2ca0

deliverables/wallaby/horizon.yaml

@@ -35,6 +35,10 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: beed6bf6f6f83df9972db5fb539d64175ce12ce9
+  - version: wallaby-em
+    projects:
+      - repo: openstack/horizon
+        hash: beed6bf6f6f83df9972db5fb539d64175ce12ce9
 branches:
   - name: stable/wallaby
     location: 19.1.0