First batch of wrong max_seats_used cleanup was done by a previous MR
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78107.
This MR is the second batch, which aims to clean up all remaining ones.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78107 has covered
gitlab_subscriptions with start_date >= 02-Aug-2021. This MR covers
gitlab_subscriptions with start_date < 02-Aug-2021.

Changelog: fixed