doc: update 2fa information in onboarding.md

Because it is no longer possible to add someone to the nodejs GitHub
organization if they do not have two-factor authentication enabled,
update the text of the onboarding documentatino to reflect this.

PR-URL: https://github.com/nodejs/node/pull/18968


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matheus Marchini <matheus@sthima.com>