doc: sync security policy with nodejs.org

The Node.js security disclosure policy has diverged between the website
and github:
- https://nodejs.org/en/security/
- https://github.com/nodejs/node/security/policy

The website is more recent and accurate, so sync the content from:
- https://github.com/nodejs/nodejs.org/blob/master/locale/en/security.md

PR-URL: https://github.com/nodejs/node/pull/29682


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>