Add MySqlSslMode.Disabled

[bgrainger]

https://dev.mysql.com/doc/relnotes/connector-net/en/news-8-0-29.html

• The SslMode=none connection-option value is deprecated (marked obsolete) and is replaced with the SslMode=disabled connection-option value.

Add Disabled as an alias for None.

[bgrainger]

Added in 2.1.9.