Enable assign plugin for openshift-eng/cyborg (#78111)

The assign plugin was removed during a plugin config synchronization
in April 2024. This re-enables it so that /cc and /assign commands
work on the repository.

Author: hoxhaeris

core-services/prow/02_config/openshift-eng/cyborg/_pluginconfig.yaml

@@ -19,6 +19,7 @@ plugins:
   openshift-eng/cyborg:
     plugins:
     - approve
+    - assign
     - blunderbuss
     - hold
     - label