[POSTGRESQL] az postgres flexible-server long-term-retention: Add breaking change announcement for command group removal (#32916)

Author: nasc17

src/azure-cli/azure/cli/command_modules/postgresql/_breaking_change.py

@@ -17,6 +17,11 @@
                                  'other automatically generated recommendations which are covered by the '
                                  'new command.')
 
+# Long term retention  command argument changes
+register_command_group_deprecate(command_group='postgres flexible-server long-term-retention',
+                                 message='Long term retention command group will be removed. '
+                                 'For more information, open a support incident.')
+
 # Upgrade command argument changes
 register_other_breaking_change('postgres flexible-server upgrade',
                                message='The allowed values will be changed from set list to '