In https://gitlab.com/gitlab-org/gitlab/-/issues/334132 it was
identified that the `max_seats_used` of `GitlabSubscription` was not
correctly resetting when a new term starts (e.g. subscription renewal)

The wrong resetting logic was fixed by
https://gitlab.com/gitlab-org/gitlab/-/issues/334132.

This MR fix the existing wrong `max_seats_used` values in a background
migration script.

Changelog: fixed