Merge branch '480118-use-work_items-for-epics' into 'master'

Add changes to use work_item in epic

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/165827



Merged-by: Terri Chu <tchu@gitlab.com>
Approved-by: Vamsi Vempati <vvempati@gitlab.com>
Approved-by: Tomas Bulva <tbulva@gitlab.com>
Approved-by: Dmitry Gruzd <dgruzd@gitlab.com>
Approved-by: Terri Chu <tchu@gitlab.com>
Reviewed-by: Terri Chu <tchu@gitlab.com>
Reviewed-by: Vamsi Vempati <vvempati@gitlab.com>
Co-authored-by: Siddharth Dungarwal <sdungarwal@gitlab.com>