Add enterprise_user key to CODEOWNERS

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183267



Merged-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Approved-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Approved-by: Kev Kloss <kkloss@gitlab.com>
Approved-by: Grzegorz Bizon <grzegorz@gitlab.com>
Reviewed-by: Grzegorz Bizon <grzegorz@gitlab.com>
Reviewed-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Co-authored-by: habdul-razak <habdul-razak@gitlab.com>