Convert work_items_hierarchy FF to group actor
See #363447 (comment 1037620850)
It would be more convenient if the work_items_hierarchy
feature flag was checked against the group actor rather than project as it will now need to be tested alongside work_items
, which is at the group level.
Both of these feature flags will need to be defaulted to on
at the same time in order to release Tasks using the hierarchy widget.