doc: update backport labels in release guide

Add `backport-open-v1.x,backported-to-v1.x` labels to `branch-diff`
commands as these indicate pull requests that are being manually
backported and should not be cherry-picked.

PR-URL: https://github.com/nodejs/node/pull/39544


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>