doc: revise security-reporting text in README

Simplify and clarify the security-reporting text in the README. Now is
also probably a good time to ping the security triage folks to make sure
the text is still accurate.

PR-URL: https://github.com/nodejs/node/pull/23407


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>