doc: no longer maintain a CNA structure

Node.js hasn't touched the cve-management repo since the Feb 2019
security release, we've used the HackerOne CVE allocation process.

Maintaining our status as a CNA is not zero cost, there is some routine
adminstration that is requested (see this doc for details).

As we no longer use the CVE management process, I propose removing it.
If this lands, I will go through the interactions with Mitre so that
Node.js is no longer a CNA and cleanup related resources (email aliases,
archive the cve-management repo, whatever else I find).

PR-URL: https://github.com/nodejs/node/pull/33639


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>