[magnum] Create 2023.2 branch for client and non-client libs

This creates the stable/2023.2 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, 08.

Change-Id: I0ad98e70e6abbcb1c60de67478965e4b64559ff8

deliverables/bobcat/python-magnumclient.yaml

@@ -11,3 +11,6 @@ releases:
     projects:
       - repo: openstack/python-magnumclient
         hash: ef63e2e1bb063f285fd669021dd70fbbbaca95f0
+branches:
+  - name: stable/2023.2
+    location: 4.2.0