build: dont-land gyp-next PRs on LTS branches

gyp-next doesn't support Python 3 anymore so we can't backport its
updates.

PR-URL: https://github.com/nodejs/node/pull/38887


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>