Prevent posts to MRs to explode gitlab note size limit
If a MR is used too many times in the nightly builds, the message with the list of links to the builds grows beyond the limit imposed by Gitlab on the message size, resulting in the error:
gitlab.exceptions.GitlabUpdateError: 400: 400 Bad request - Note {:note=>["is too long (maximum is 1000000 characters)"]}(on 2025-03-22 it became extremely disruptive because the problematic MR was in PRConfig, included in all slots).
With this change the messages are not allowed to grow too much by creating new messages when we pass a threshold.