Replace the uses of epic create service with work item service

After the sync from WorkItem to Epic is completed we can replace the uses of Epics::CreateService and Epics::UpdateService with work item services. This change will prevent further writing to the unionized associations and allow us to remove part of the legacy epic code.

The rest of the issues in Workstream 5: Delegate writing shared associati... (&14079) may be redundant after this change, but we will need to check case-by-case to be sure.