Replace flag synced_epic_work_item_editable with work_item_epics

What does this MR do and why?

Related to #467686 (closed)

Replace the use of synced_epic_work_item_editable feature flag with work_item_epics.

This is part of the work for consolidating all feature flags involved in work item epics. There are no changes in behaviour, only that everything that was guarded by synced_epic_work_item_editable is now guarded by the flag work_item_epics.

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