Mention CardDAV rather than GroupDAV as supported protocol

GroupDAV has been superseded by CalDAV/CardDAV more than a decade ago.

I've also looked into deprecating that entirely for 6 (driven by the XQuery
use in KDAV), all our supported groupware servers have long switched,
except one, Citadel. They are still stuck on GroupDAV exclusively. So
I guess we have to drag this along for another decade...