Update the failure message in CherryPick::StableNotifier
The previous version of this message suggested one possible fix was to "resolve the conflicts against the branch and re-add the label". Because the MR is always merged at this point, this is impossible.
Instead we only suggest one solution: opening a new MR targeting the branch of the preparation MR.
Closes https://gitlab.com/gitlab-org/release-tools/-/issues/456