[magnum] Create 2023.1 branch for client and non-client libs

This creates the stable/2023.1 branch for all client and non-client
libraries now that we are past the library freeze.

If the team wants to wait for a specific patch to make it to the
library (for example if the gate of the deliverable is broken and
needs to be fixed before branching), then someone from the team can
-1 the patch to have it held, or update that patch with a different
commit SHA.

Patches with no response will be merged by Friday, February 24th.

Change-Id: Ie0ace6abdf332702d5d4c5792d8aab1840786da0

deliverables/antelope/python-magnumclient.yaml

@@ -11,3 +11,6 @@ releases:
     projects:
       - repo: openstack/python-magnumclient
         hash: de11f40d0c632ff839e5838ed650d1821c87d8a8
+branches:
+  - name: stable/2023.1
+    location: 4.1.0