WIP: [EE specific parts] `disable_statement_timeout` will no longer leak to other migrations

What does this MR do?

This is the EE specific parts fixes required after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20503 is merged

Are there points in the code the reviewer needs to double check?

Check https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20503 first. This is here to introduce fixes to the remaining migrations exclusive to EE.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/48967 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20503