doc: synch SECURITY.md with website

Refs: https://github.com/nodejs/nodejs.org/pull/3106#issuecomment-614258785

PR-URL: https://github.com/nodejs/node/pull/32903


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>