migrate to Swift 6.2 / swift-binary-parsing

[lhoward]

Migrating to Swift 6.2 will allow us to use the new Span API and improve deserialisation efficiency for PDUs that are decoded directly (i.e. not using Codable).

We won't do this until Swift 6.2 is released.

[lhoward]

I think, we probably still want Swift 5 compatibility for macOS / iOS clients so I'm not inclined to do this just yet.