Use gitlab-ruby-shadow gem instead of shadow source

See merge request !7418



Merged-by: Balasankar 'Balu' C <balasankar@gitlab.com>
Approved-by: Clemens Beck <cbeck@gitlab.com>
Approved-by: Balasankar 'Balu' C <balasankar@gitlab.com>
Reviewed-by: Clemens Beck <cbeck@gitlab.com>
Co-authored-by: Stan Hu <stanhu@gmail.com>