Merge branch 'add-seat-control-user-cap-validation' into 'master'

Add Validation to namespace_settings.new_user_signups_cap

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159959



Merged-by: Suraj Tripathi <stripathi@gitlab.com>
Approved-by: Suraj Tripathi <stripathi@gitlab.com>
Reviewed-by: Vijay Hawoldar <vhawoldar@gitlab.com>
Co-authored-by: Jason Goodman <jgoodman@gitlab.com>