Namespace API expose GitlabSubscription.end_date

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137847



Merged-by: Etienne Baqué <ebaque@gitlab.com>
Approved-by: Paulo Barros <pbarros@gitlab.com>
Approved-by: Etienne Baqué <ebaque@gitlab.com>
Reviewed-by: Etienne Baqué <ebaque@gitlab.com>
Reviewed-by: Paulo Barros <pbarros@gitlab.com>
Co-authored-by: Qingyu Zhao <qzhao@gitlab.com>