Fix incorrect max_seats_used values

What does this MR do and why?

For issue #345894+

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.

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Related to #345894