Neutron: Retire networking-ovn

The last stable branch of networking-ovn, stable/train, was already
retired[1] and the project no longer has any active stable branches,
so can be formally retired.

[1] https://review.opendev.org/c/openstack/releases/+/865967

Change-Id: I0f3e1fda9080b3a433783ae2c423f82f638a5457

reference/legacy.yaml

@@ -577,6 +577,10 @@ neutron:
       retired-on: 2016-10-31
       repos:
         - openstack/python-neutron-pd-driver
+    networking-ovn:
+      retired-on: 2024-05-15
+      repos:
+        - openstack/networking-ovn
 
 octavia:
   irc-channel: openstack-lbaas

reference/projects.yaml

@@ -939,11 +939,6 @@ neutron:
       release-management: deprecated
       repos:
         - openstack/networking-odl
-    networking-ovn:
-      deprecated: ussuri
-      release-management: deprecated
-      repos:
-        - openstack/networking-ovn
     networking-sfc:
       repos:
         - openstack/networking-sfc