WorkItems: Update Rolledup dates when a child milestone is updated

As part of the [epic], we need to ensure that when a child milestone is
updated, in a WorkItem hierarchy we update all the parent WorkItems in
that hierarchy with _rolledup dates_ to use the most broad start/due
dates in its hierarchy.

This is being developed behind the feature flag: `work_items_rolledup_dates`

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/434835

Changelog: added
EE: true

[epic]: https://gitlab.com/groups/gitlab-org/-/epics/11409

ee/app/workers/work_items/rolledup_dates/update_parent_rolledup_dates_event_handler.rb

@@ -12,6 +12,8 @@ class UpdateParentRolledupDatesEventHandler
       UPDATE_TRIGGER_ATTRIBUTES = %w[
         start_date
         due_date
+        milestone
+        milestone_id
       ].freeze
 
       UPDATE_TRIGGER_WIDGETS = %w[