Upgrade MQTTNet to latest version, add support for .NET 5

[duizendnegen]

We are running .NET 5 with our Azure Functions; this PR updates this project to support that.

[duizendnegen]

Some WIP on our side: we're still testing it with a live MQTT queue, so not 100% ready to merge yet.

[pseabury]

@keesschollaart81 can this be merged and possibly updated for NET6 and latest MQTTNet package?

[keesschollaart81]

Will look into this next weekend. LGTM!

[duizendnegen]

Sorry to say that we didn't get this working on our end; I would recommend against merging as-is. Skipping .NET 5 and going straight to .NET 6 will most likely give you a much better experience for upgrading.

[keesschollaart81]

Good that you're report this! What was the issue / should I look out for?

[duizendnegen]

Honestly; actually receiving a message from a test queue. I don't remember exactly because it's been too long ago. The different modes of running it isolated / non-isolated ended up biting us in the rear as far as I remember.

[pseabury]

@duizendnegen I'm most interested in running NET6 Isolated - actually about to try it - as Microsoft claims it's the future of Functions. Do you recall if isolated was the problem?

[pseabury]

I tried to start porting to NET6/Isolated but as you probably know configuration & bindings are way different. I wasn't able to complete a working version of this library yet.

[duizendnegen]

I had a similar issue; configurations / bindings being way different.