Deprecate the OpenPGP support

Currently we provide limited support for establishing connections with OpenPGP certificates and keys. It is limited because it has no specific use-cases that it addresses, and mostly serves as a technology preview. As the OpenPGP certificate for HTTPS (or TLS in general) never got any traction and GnuTLS is the only implementation supporting it, we should deprecate that code and drop it completely when an opportunity is given.

This relates loosely to #26 (closed). The only known use case for OpenPGP certificates is when one would like to avoid relying on the whole X.509 PKI.