Merge branch '467686-consolidate-feature-flags-for-work-items-epics-2' into 'master'

Replace flag synced_epic_work_item_editable with work_item_epics

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/158921



Merged-by: Nicolas Dular <ndular@gitlab.com>
Approved-by: Alexandru Croitor <acroitor@gitlab.com>
Approved-by: Alex Buijs <abuijs@gitlab.com>
Reviewed-by: Nicolas Dular <ndular@gitlab.com>
Co-authored-by: Eugenia Grieff <egrieff@gitlab.com>