Allow vulnerability management policy in update policy mutation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/166939



Merged-by: Sashi Kumar Kumaresan <skumar@gitlab.com>
Approved-by: Michał Zając <mzajac@gitlab.com>
Approved-by: Sashi Kumar Kumaresan <skumar@gitlab.com>
Co-authored-by: Lorenz van Herwaarden <lvanherwaarden@gitlab.com>