Adds a note to not re-assign to the bot until MRs are ready
What does this MR do?
Some backports were accidentally missed when we did the merge in https://gitlab.com/gitlab-org/release/retrospectives/-/issues/33. https://gitlab.com/gitlab-org/security/gitlab/-/issues/34#note_310776812 .
This adds a warning not to re-assign to the bot until all backports are ready to avoid this situation, which is a temporary workaround until https://gitlab.com/gitlab-org/release-tools/-/issues/369 is complete.