PGP: Replace obsolete XEP-0027 by (experimental) XEP-0373
From https://xmpp.org/extensions/xep-0373.html#solving-xep0027-issues
This specification [XEP-0373] addresses all relevant issues of Current Jabber OpenPGP Usage (XEP-0027). It mitigates replay attacks by including the recipient's address and a timestamp in the OpenPGP content element. It allows for both, signing and encrypting of the element.