Fix `updated_at` doesn't apply to `state_event` updates of issues via API
What does this MR do?
Continued from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23860.
When updated_at is overridden through the API, use that timestamp for any system notes that come out of the update too.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46980.