Merge branch 'assignee-widget-to-callback' into 'master'

Refactor assignees widget service to new widget architecture

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/140001



Merged-by: Jessie Young <jessieyoung@gitlab.com>
Approved-by: Heinrich Lee Yu <heinrich@gitlab.com>
Reviewed-by: Donald Cook <dcook@gitlab.com>
Co-authored-by: Shubham Kumar <shubhamkrai123@gmail.com>