Update plugin for 5.6

[TimothyBJacobs]

We need to update the plugin so that when it is running on WordPress 5.6 it doesn't output its UI and doesn't register the authentication actions. Right now, the UI is pretty much completely broken with the plugin active on 5.6.

I think we probably want to keep the existing REST API and the API methods on Application_Passwords but with a _deprecated_function call.