advertising SCRAM-SHA-256 breaks irccloud and possibly other clients

[slingamn]

If a server advertises both PLAIN and SCRAM-SHA-256, irccloud attempts SCRAM, but if it fails it doesn't fall back to PLAIN. This breaks authentication for anyone who registered an account on Ergo 2.7 or lower --- we need at least one successful PLAIN authentication in order to generate the SCRAM credentials.

Accordingly, advertisement of SCRAM-SHA-256 will be disabled in Ergo 2.8; we'll revisit down the line.