Merge branch '493977-responsive-throttling' into 'master'

Add responsive throttling for placeholder user records reassignment

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/173292



Merged-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Approved-by: Luke Duncalfe <lduncalfe@gitlab.com>
Approved-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Reviewed-by: Luke Duncalfe <lduncalfe@gitlab.com>
Reviewed-by: Krasimir Angelov <kangelov@gitlab.com>
Co-authored-by: carlad-gl <cdrago@gitlab.com>