doc: simplify security reporting text

Edit security-reporting text in the README to keep it concise and
straightforward. The removed text may discourage reporting. Nothing like
it appears in similar security-reporting text that I have reviewed.
See, for example, the Linux kernel docs on security reporting:
https://www.kernel.org/doc/html/v4.11/admin-guide/security-bugs.html

PR-URL: https://github.com/nodejs/node/pull/23686


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