Make MySqlConnectorLogManager.Provider [Obsolete]

[bgrainger]

MySqlConnectorLogManager.Provider is "deprecated" in MySqlConnector 2.3.0:

MySqlConnector/docs/content/overview/version-history.md

Line 43 in 413b4bb

* `MySqlConnectorLogManager.Provider` can still be used to add a logging destination, but it is now deprecated.

However, the property hasn't been marked [Obsolete] in code:

MySqlConnector/src/MySqlConnector/Logging/MySqlConnectorLogManager.cs

Line 15 in 413b4bb

public static IMySqlConnectorLoggerProvider Provider

In a future release (maybe 2.4.0?), this property should be marked [Obsolete] with more direct guidance to users to use AddMySqlDataSource or UseLoggerFactory.