Merge branch 'move_epic_as_work_item_concern_to_issue_ee' into 'master'

Move WorkItems::EpicAsWorkItem to EE:Issue model

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167303



Merged-by: Mario Celi <mcelicalderon@gitlab.com>
Approved-by: Mario Celi <mcelicalderon@gitlab.com>
Reviewed-by: Alexandru Croitor <acroitor@gitlab.com>
Reviewed-by: Nicolas Dular <ndular@gitlab.com>
Reviewed-by: Mario Celi <mcelicalderon@gitlab.com>
Co-authored-by: Alexandru Croitor <acroitor@gitlab.com>
Co-authored-by: Nicolas Dular <ndular@gitlab.com>