Add availability requirements to NIOAsyncSequenceProducer extension (#2236)

Motivation:

One of the `NIOAsyncSequenceProducer` extensions was missing a
availability requirements.

Modifications:

Add missing requirement.

Result:

Fewer bugs.

Author: glbrntt

Sources/NIOCore/AsyncSequences/NIOAsyncSequenceProducer.swift

@@ -1068,6 +1068,7 @@ extension NIOAsyncSequenceProducer {
 
 /// The ``NIOAsyncSequenceProducer/AsyncIterator`` MUST NOT be shared across `Task`s. With marking this as
 /// unavailable we are explicitly declaring this.
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 @available(*, unavailable)
 extension NIOAsyncSequenceProducer.AsyncIterator: Sendable {}
 #endif