Update znc.in/server-time-iso capability name

[kgraefe]

Since ZNC 1.7.0 znc.in/server-time-iso has been renamed to server-time. The old name is still valid but will be removed eventually. This is 5 years now, so I think we can safely assume that everyone updated their server.

Maybe we should also rework capability negotiation: https://ircv3.net/specs/extensions/capability-negotiation.html (Note that Pidgin may send CAP END when SASL is built-in and enabled...)