Replace MONO define with standard __MonoCS__

StackExchange.Redis/StackExchange/Redis/ConnectionMultiplexer.cs

@@ -169,7 +169,7 @@ internal void OnErrorMessage(EndPoint endpoint, string message)
         static void Write<T>(ZipArchive zip, string name, Task task, Action<T, StreamWriter> callback)
         {
             var entry = zip.CreateEntry(name,
-#if MONO
+#if __MonoCS__
                 CompressionLevel.Fastest
 #else
                 CompressionLevel.Optimal

StackExchange.Redis/StackExchange/Redis/SocketManager.NoPoll.cs

-#if MONO
+#if __MonoCS__
 
 namespace StackExchange.Redis
 {

StackExchange.Redis/StackExchange/Redis/SocketManager.Poll.cs

@@ -5,7 +5,7 @@
 using System.Runtime.InteropServices;
 using System.Threading;
 
-#if !MONO
+#if !__MonoCS__
 namespace StackExchange.Redis
 {