Implement the benefits of switching to Swift 6

[lawrence-forooghian]

Requires #21 first.

• do Don’t create a Logger instance each time we need to log #40
• use AsyncSequence with primary associated types, allowing us to remove our complicated Subscription type and replace it with things like any AsyncSequence<Message, Never>
• update the mocks of Create a mock implementation of the public API, for example app #4 to use primary associated types too
• switch RoomLifecycleContributorChannel to use typed throws

┆Issue is synchronized with this Jira Task by Unito

[lawrence-forooghian]

Hmm, I have a feeling that we aren’t going to be able to implement the AsyncSequence improvements given above; it seems like:

• using AsyncSequence with primary associated types requires macOS 15 etc (gives the error 'Failure' is only available in macOS 15.0 or newer)
• specifying that an AsyncSequence is unable to throw (e.g. by writing associatedtype Subscription: AsyncSequence where Subscription.Element == PresenceEvent, Subscription.Failure == Never) gives the same error.

So, I think I’ll close this issue, and update the comments in the code that relate to this improvement. Maybe I’m wrong and there is something that we can improve, but it's not obvious to me at the moment 🤷