Fully move to paho-mqtt v2 APIs #305
Description
We've updated this library to work for both v1 and v2, but at some point we can stop supporting v1.
The main change for v2 we should consider is https://eclipse.dev/paho/files/paho.mqtt.python/html/migrations.html#versioned-the-user-callbacks -- passing this argument is not supported in v1, but we can start passing it and upgrade to the new callbacks at some point, perhaps after a few upstream releases.
Filing this issue to track the current state.