Use Group.workItems query for Roadmap app once new epic experience is defaulted

Summary

With Roadmap frontend app (i.e. ee/app/assets/javascripts/roadmap) already refactored to use VueApollo as a part of https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/6560, once we fully migrate to Epic Work Items, we need to replace Group.epics query with Group.workItems query (provided that it supports everything around filters, searching, and sorting as Group.epics query).

This requires significant changes to the Roadmap frontend to be able to handle the new data structure but visually, it should cause no impact to how Roadmap app looks and behaves.