Add missing availability (#1952)

Tests/GRPCHTTP2TransportTests/Test Utilities/HTTP2StatusCodeServer.swift

@@ -23,6 +23,7 @@ import NIOPosix
 /// An HTTP/2 test server which only responds to request headers by sending response headers and
 /// then closing. Each stream will be closed with the ":status" set to the value of the
 /// "response-status" header field in the request headers.
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 final class HTTP2StatusCodeServer {
   private let address: EventLoopPromise<GRPCHTTP2Core.SocketAddress.IPv4>
   private let eventLoopGroup: MultiThreadedEventLoopGroup