Check work_item_epics FF for legacy date rollup

What does this MR do and why?

Check work_item_epics FF for legacy date rollup

When the hierarchy is not correctly synced to the work item side, the rolledup date calculation would be not correct.

Since the feature flag work_items_rolledup_dates is already by default enabled, this could lead to dates not being rolled up correctly.

For this reason we add another feature flag check for work_item_epics which is not yet enabled by default.

Changelog: fixed EE: true

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