Introduce security@roundcube.net as security contact

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.

SECURITY.md

@@ -10,7 +10,7 @@ If you found a security issue or vulnerability of the software, please report it
 
 Your report should include clear steps for reproduction and a classification of the found vulnerability.
 
-You can also send an encrypted email message to *alec[at]alec.pl*. You can find the PGP public key on the major public keyservers like [pgp.key-server.io](https://pgp.key-server.io).
+If you prefer, you can also send an encrypted email message to `security [at] roundcube.net`. The [PGP key](https://roundcube.net/download/security.roundcube.net.pub)'s fingerprint is `ACFCF63232B79518E632EC4B0127B799F939816F`.
 
 ## Publishing and Credits