Deprecate oauth/ro endpoint

[joshcanhelp]

This is unlikely to affect anyone but the methods that use this endpoint are public so we'll need to properly deprecate these (E_DEPRECATION error and documentation in the code). Since these are not used in any of the core processes, this will only affect developers who have extended the plugin to use this endpoint.

Affected methods:

WP_Auth0_Api_Client::ro() - posts to the oauth/ro endpoint and returns a decoded response body. More information here, replace with a Password Grant.

WP_Auth0_LoginManager::login_with_credentials() - calls WP_Auth0_Api_Client::ro().

These will be deprecated in the next version and removed sometime this year. If you have any questions or need any assistance migrating to the new flow, please let us know in the comments below or on our Community site.

[joshcanhelp]

Fixed with #413