Merge branch '377588-work-item-header-ui-improvements' into 'master'

Work item header and input UI improvements

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126425



Merged-by: Coung Ngo <cngo@gitlab.com>
Approved-by: Deepika Guliani <dguliani@gitlab.com>
Approved-by: Coung Ngo <cngo@gitlab.com>
Reviewed-by: Simon Knox <simon@gitlab.com>
Co-authored-by: Simon Knox <simon@gitlab.com>