Downgrade paho-mqtt to 1.x

[deviantintegral]

Summary

paho-mqtt released a new 2.x version last week, and we weren't pinning or specifying a specific version in the docker scripts.

Thanks to the report at fl4p/batmon-ha#197 for confirming this.

I also snuck in some basic help message improvements for when the mqtt addon itself isn't available.

Alternatives Considered

I'll file an upstream issue since this will break the out-of-the-box experience for the rtl_433 project too.

Testing Steps

1. Pull ghcr.io/pbkhrv/rtl_433-hass-addons-rtl_433_mqtt_autodiscovery-next-{arch}:180-merge once builds are done.
2. Tag it locally as next or the 0.8.0 release to overwrite it locally.
3. Addon should start.