Cleanup all warnings in 5.9 build

[ktoso]

We have a lot of self induced "deprecation warnings" because we use the runtime that people should not be touching.

• We should probably remove a bunch of those deprecations because they are not types the public API exposes and we just have the noise to suffer through it.
• We should consistently remove those deprecated "actor ref" APIs
• We should address new warnings that appear in the 5.9 build

[ktoso]

There's a lot of these annoying ones:

/Users/ktoso/code/swift-distributed-actors/Sources/DistributedCluster/ClusterSystem.swift:833:38: warning: conformance of 'Never' to 'Encodable' is only available in macOS 14.0 or newer
        guard let selector = context.selectorSegments.first else {
                                     ^
/Users/ktoso/code/swift-distributed-actors/Sources/DistributedCluster/ClusterSystem.swift:833:38: note: add 'if #available' version check
        guard let selector = context.selectorSegments.first else {
                                     ^
/Users/ktoso/code/swift-distributed-actors/Sources/DistributedCluster/ClusterSystem.swift:832:10: note: add @available attribute to enclosing instance method
    func _resolveUntyped(context: _ResolveContext<Never>) -> _AddressableActorRef {
         ^

due to Never gaining a Codable conformance as well