Data integrity on approvals_required

It was expected that the data migration would ensure MR approval_rules' approvals_required would not be lower than that of the project's rules. However in the recent query on production data some cases were found, see:

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9267#note_133936022

This would not be a big issue as on application level a max call is there to ensure we use the bigger of the values. However it is worth checking out what went wrong.