[Auth] Fix FIRGameCenterAuthProvider warning on Xcode 13 (#10809)

The method "fetchItemsForIdentityVerificationSignature:" is marked as available only on tvOS 13.5+ in Xcode 13 (tvOS 15 SDK) but was extended to tvOS 13.4.8+ in Xcode 14 (tvOS 16 SDK).

Updated the availability check to limit to tvOS 13.5+ when compiling with Xcode 13.

#no-changelog

Author: andrewheard

FirebaseAuth/Sources/AuthProvider/GameCenter/FIRGameCenterAuthProvider.m

@@ -54,7 +54,16 @@ + (void)getCredentialWithCompletion:(FIRGameCenterCredentialCallback)completion
     }
     return;
   }
-  if (@available(iOS 13.5, macOS 10.15.5, macCatalyst 13.5, tvOS 13.4.8, *)) {
+  if (@available(iOS 13.5, macOS 10.15.5, macCatalyst 13.5,
+  // Availability of fetchItemsForIdentityVerificationSignature was extended to tvOS 13.4.8 in
+  // Xcode 14 (from tvOS 13.5 in Xcode 13).
+  // TODO: Remove this check when we no longer support Xcode 13.
+#if defined(__TVOS_16_0)
+                 tvOS 13.4.8,
+#else
+                 tvOS 13.5,
+#endif
+                     *)) {
     [localPlayer fetchItemsForIdentityVerificationSignature:^(
                      NSURL *_Nullable publicKeyURL, NSData *_Nullable signature,
                      NSData *_Nullable salt, uint64_t timestamp, NSError *_Nullable error) {