Add availability to ConstantAsyncSequence (#107)

We were running into build issues like this

>
grpc-swift-nio-transport/Sources/GRPCNIOTransportCore/Internal/ConstantAsyncSequence.swift:37:17:
error: concurrency is only available in macOS 10.15.0 or newer

when building this package on macOS. It looks like there is just a
missing availability annotation.

Author: FranzBusch

Sources/GRPCNIOTransportCore/Internal/ConstantAsyncSequence.swift

@@ -16,6 +16,7 @@
 
 internal import GRPCCore
 
+@available(gRPCSwiftNIOTransport 1.0, *)
 private struct ConstantAsyncSequence<Element: Sendable>: AsyncSequence, Sendable {
   private let element: Element