Using a dedicated email address with a dedicated PGP key allows to give
multiple people access while still keeping things under wrap.
A single, private email address as security contact is such a huge bus
factor, which we should avoid. Event just a holiday or illness could
lead to escalation due to missing replies.
Also, in case of potentially severe security issues Nextcloud's security
team must have access to all details and communication. This is already
given for all issues reported via hackerone.com, and with this change is
now also enabled for issues reported by email.