Make Customer object state consistent with related GitLab User object

Problem

From this thread: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/7089#note_1485419846

CustomersDot can behave in isolation when it comes to altering Customer objects. It is possible to then invalidate a Customer object while having the related User object on GitLab to remain active.

Proposal

Before an account on CustomersDot becomes inactive, check first whether its related User object is still active on GitLab.

To put it another way: should the state of a User object on GitLab drive the state of CustomersDot's Customer object?