Add missing availability (#1936)

Tests/GRPCHTTP2TransportTests/ControlService.swift

@@ -18,6 +18,7 @@ import GRPCCore
 
 import struct Foundation.Data
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 struct ControlService: ControlStreamingServiceProtocol {
   func unary(
     request: ServerRequest.Stream<Control.Method.Unary.Input>
@@ -44,6 +45,7 @@ struct ControlService: ControlStreamingServiceProtocol {
   }
 }
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 extension ControlService {
   private func handle(
     request: ServerRequest.Stream<ControlInput>