Concurrency warning in example server: Conformance of 'NIOSSHHandler' to 'Sendable' is unavailable

I'm experimenting with porting a simple server app I'm working on from the (quite nice) Citadel wrapper library to use `NIOSSH` directly. I've got it basically working, but I get a concurrency warning that says it will be an error in Swift 6 mode.

It seems that the `ChannelPipeline.addHandler` method used in the [example server app](https://github.com/apple/swift-nio-ssh/blob/main/Sources/NIOSSHServer/main.swift#L89) wants its `handlers` input to be `[ChannelHandler & Sendable]`, but the `NIOSSHHandler` type is [explicitly marked as non-`Sendable`](https://github.com/apple/swift-nio-ssh/blob/main/Sources/NIOSSH/NIOSSHHandler.swift#L91-L92).

It looks like there were some updates to that type related to strict concurrency in https://github.com/apple/swift-nio-ssh/pull/196 - is it possible that the type is now effectively `Sendable` if the availability annotation were removed? I don't know enough about the internals here to have a clue, but perhaps a maintainer would.

Start with Sources/NIOSSHServer/main.swift around the example server's ChannelPipeline.addHandler call and Sources/NIOSSH/NIOSSHHandler.swift around its Sendable annotation. Review the strict-concurrency changes from pull request 196 and determine whether the example can use NIOSSHHandler without the Swift 6 warning. Done means the example builds without this warning and the handler's concurrency declaration remains correct.

Written by the indexing model from the issue text.