Clarify documentation on sha256_password compatibility

The public documentation for mariadb connector/j states that the sha256_password authentication mode is supported from version 2.5.0 onwards.

https://mariadb.com/kb/en/authentication-plugin-sha-256/#using-plugins-with-mariadb-connectorj

What it doesn't mention is that this method was removed in version 3+ and is no longer available.

As discussed in https://jira.mariadb.org/browse/CONJ-1114, it appears this is a deliberate choice and the plugin isn't going to be reinstated. However, this should be clarified in the documentation and users advised to switch to different authentication modes.