refactor: upgrade ILPP backend, drop 2019.4 support, rename types/fields (#895)

Author: 0xFA11

com.unity.multiplayer.mlapi/Editor/CodeGen/CodeGenHelpers.cs

@@ -13,10 +13,6 @@
 using Unity.CompilationPipeline.Common.ILPostProcessing;
 using UnityEngine;
 
-#if !UNITY_2019_4_OR_NEWER
-#error MLAPI requires Unity 2019.4 or newer
-#endif
-
 namespace MLAPI.Editor.CodeGen
 {
     internal static class CodeGenHelpers