Rename feature flag for epic to work item sync

This renames the feature flag from `epic_creation_with_synced_work_item`
to `sync_epic_to_work_item`. This has the following reasons:

1. We not only create work items when an epic gets created, but sync
   updates as well.
2. We want to sync the other way around in the future (work item to
   epic) where we will create the opposite feature flag and want to have
   a consistent naming.