Replace custom mono defines with toolchain-provided defines.

OpenRA.Game/Support/AssemblyLoader.cs

@@ -11,7 +11,7 @@
 
 // Not used/usable on Mono. Only used for Dotnet Core.
 // Based on https://github.com/natemcmaster/DotNetCorePlugins and used under the terms of the Apache 2.0 license
-#if !MONO
+#if NET5_0_OR_GREATER
 using System;
 using System.Collections.Generic;
 using System.IO;

OpenRA.Game/Support/HttpClientFactory.cs

@@ -16,7 +16,7 @@ namespace OpenRA.Support
 {
 	public class HttpClientFactory
 	{
-#if !MONO
+#if NET5_0_OR_GREATER
 		const int MaxConnectionPerServer = 20;
 		static readonly TimeSpan ConnectionLifeTime = TimeSpan.FromMinutes(1);
 #endif
@@ -30,7 +30,7 @@ namespace OpenRA.Support
 
 		static HttpMessageHandler GetHandler()
 		{
-#if !MONO
+#if NET5_0_OR_GREATER
 			return new SocketsHttpHandler
 			{
 				// https://github.com/dotnet/corefx/issues/26895