Clarify requests to backport to versions older than two releases

Follow up attempting to address a little bit of ambiguity https://gitlab.com/gitlab-org/release/tasks/-/issues/2998#note_708089735.

For example, a bug may be severe but not enough by itself to trigger the creation of a patch release 3 versions old. But you wish to request to include your fix, in the event that some other bug happens to trigger the creation of a patch release 3 versions old.