WL#12694: Deprecate the non-caching sha256 authentication plugin

Caching SHA-2 Pluggable Authentication is a superset of  SHA-256 Pluggable
Authentication. 

As such (and also because it's now the new default authentication method) it
doesn't make sense to keep sha-256 anymore.

This worklog is about deprecating sha256 as both a server side authentication
plugin (a deprecation warning will be issued when used) and as a client side
authentication plugin (since the libmysql client library cannot generate
warnings by itself it'll be a documentation only add-on). 

Caching SHA-2 authentication plug-in uses caching on the server side for better
performance 
and has additional features for wider applicability. 
Caching SHA-2 authentication plug-in is a super set of SHA-256 authentication
plug-in.
SHA-256 authentication plug-in will be deprecated.

MySQL server will give deprecate message when SHA-256 authentication method is used.

FR 1.1: MySQL server will give deprecate message in the server log when user
with SHA-256 authentication method try to authenticate.

FR 1.2: MySQL server will give deprecate message in the server log when
CREATE/ALTER USER with sha256_password SQL command is used.

NFR 1: For MySQL clients, SHA-256 authentication method deprecate message will
be documented.

NFR 2: None of the sha256_password deprecate message will be shown in the MySQL
console.