Migrate Paho MQTT client to Callback API version 2

[rgrizzell]

paho-mqtt recently updated their Callback API to version 2. Using their provided migration guide, this pull request addresses some issues with newer installations of meshtastic-bridge.

• Resolves the deprecation warning on start-up

  /home/rgrizzell/meshtastic-bridge/main.py:129: DeprecationWarning: Callback API version 1 is deprecated, update to the latest version
    mqttc = mqtt.Client()
  

• Fixes an issue where the MQTT client fails to start when a client_id is specified in config.yaml.

  Traceback (most recent call last):
    File "/home/rgrizzell/meshtastic-bridge/main.py", line 127, in <module>
      mqttc = mqtt.Client(client_id)
    File "/home/rgrizzell/meshtastic-bridge/lib64/python3.10/site-packages/paho/mqtt/client.py", line 772, in __init__
      raise ValueError(
  ValueError: Unsupported callback API version: version 2.0 added a callback_api_version, see docs/migrations.rst for details
  

• Improves error handling around MQTT Auth and ACL issues.

  ERROR:meshtastic.bridge:Could not connect to MQTT local [Not authorized]