Increase scan purge iteration size to keep up with scan rate

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183707



Merged-by: Simon Tomlinson <stomlinson@gitlab.com>
Approved-by: Mehmet Emin INAC <minac@gitlab.com>
Approved-by: Simon Tomlinson <stomlinson@gitlab.com>
Reviewed-by: Mehmet Emin INAC <minac@gitlab.com>
Reviewed-by: Simon Tomlinson <stomlinson@gitlab.com>
Co-authored-by: Gregory <11164960-ghavenga@users.noreply.gitlab.com>