Change MQTTError into a struct

[adam-fowler]

This requires a major version upgrade

[adam-fowler]

Can't easily do this and keep the associated values. Perhaps we just wait for extensible enums

[fpseverino]

Perhaps we just wait for extensible enums

IIRC they should already be available in Swift 6.2.3, we could use swift-tools-version:6.2.3. Otherwise we wait for Swift 6.3

[adam-fowler]

I think we should wait for 6.3.