Use a method for updating groups in the API

Similar to creating groups in the groups API, EE injects custom code
just before we update an existing group. Moving the update logic to a
separate method in CE allows EE to more easily extend it.

lib/api/groups.rb

@@ -66,6 +66,14 @@ module API
           .execute
       end
 
+      def update_group(group)
+        # This is a separate method so that EE can extend its behaviour, without
+        # having to modify this code directly.
+        ::Groups::UpdateService
+          .new(group, current_user, declared_params(include_missing: false))
+          .execute
+      end
+
       def find_group_projects(params)
         group = find_group!(params[:id])
         options = {
@@ -161,7 +169,7 @@ module API
         group = find_group!(params[:id])
         authorize! :admin_group, group
 
-        if ::Groups::UpdateService.new(group, current_user, declared_params(include_missing: false)).execute
+        if update_group(group)
           present group, with: Entities::GroupDetail, current_user: current_user
         else
           render_validation_error!(group)