Changed name and meaning for config option that selects SASL mechanism for SIEVE.

Option com.openexchange.mail.filter.preferGSSAPI is renamed to com.openexchange.mail.filter.preferredSaslMech and no more signals a boolean flag whether to user GSSAPI or not, but now specifies the name of the concrete SASL authentication mechanism to use when connecting to SIEVE end-point. By default that option is empty.

Known values: GSSAPI, XOAUTH2, or OAUTHBEARER

An empty value falls-back to PLAIN (default).