Report expected default version String

It occurs that maxscale default to version 10.4.32 by default (https://github.com/mariadb-corporation/MaxScale/blob/214b4e92737811ee21911acf7eb111ca84f6d4a0/server/modules/protocol/MariaDB/mariadb_client.cc#L91 )

The problem is that some connectors rely on server version for some capabilities.
When dealing with xpand, with version like 5.0.45-Xpand-5.3.16, having this default to 10.4 is problematic.

For example when using java connector and query timeout, connector will then use `SET STATEMENT max_statement_time=xx FOR ` prefix for command thinking that server version is 10.1.2+, while it's not supported by server.

A better solution is for maxscale to retain last connected server version in place of using that default version.