Bumping .NET assembly version number before 6.0 release as there were changes

Source commit: 8638e314a280ccf2694cd7fd56c3d19947a4a134

dotnet/properties/AssemblyInfo.cs

@@ -35,7 +35,7 @@ namespace WinSCP
     {
         public const string UndefinedProductVersion = "9.9.9.9";
 
-        public const string Version = "1.12.0.0";
+        public const string Version = "1.13.0.0";
         public const string ProductVersion = "6.0.0.0";
     }
 }