Bumping .NET assembly version number before 1.6.4 release as there were changes

Source commit: 5ffbf860918da897d30fb842c350aa720bd45607

dotnet/properties/AssemblyInfo.cs

@@ -35,7 +35,7 @@ namespace WinSCP
     {
         public const string UndefinedProductVersion = "9.9.9.9";
 
-        public const string Version = "1.6.3.0";
+        public const string Version = "1.6.4.0";
         public const string ProductVersion = "5.15.2.0";
     }
 }