[Feature] Use package version 4.3.X of MQTTnet instead of 5.X

Currently, Brighter uses package 5.X of MQTTnet to implement its MQTT gateway. While version 5.X supports the latest version of the MQTT protocol and features, it requires at least Net 8.0.

Target Framework | MQTTnet version
-- | --
.NET 8.0 | 5
.NET 5.0 - 7.0 | 4
.NET Core 1.1+ | 4
.NET Framework 4.5.2 - 4.8.1 | 4
.NET Standard 1.3 - 2.1 | 4
Blazor WebAssembly | 5
Mono 5.2+ | 4
UWP 10.0.10240+ | 4
Xamarin (Android / iOS) | 4

My feature request is to either downgrade to using 4.3 for compatibility with more platforms or support both a v4 and v5 version of the MQTT messaging gateway.

I'm willing to work to improve the MQTT messaging gateway, as I have decided to use MQTTnet to host a broker.