Add missing availability to tests (#1890)

Co-authored-by: Gustavo Cairo <gus.cairo@apple.com>

Tests/GRPCHTTP2CoreTests/Client/HTTP2ClientTransportConfigTests.swift

@@ -17,6 +17,7 @@
 import GRPCHTTP2Core
 import XCTest
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 final class HTTP2ClientTransportConfigTests: XCTestCase {
   func testCompressionDefaults() {
     let config = HTTP2ClientTransport.Config.Compression.defaults