Bumping .NET assembly version number before 5.12.1 beta release as there were changes

Source commit: f885c1d34c917291b9e8e000987c5ad257206eb4

dotnet/properties/AssemblyInfo.cs

@@ -31,7 +31,7 @@ namespace WinSCP
     {
         public const string UndefinedProductVersion = "9.9.9.9";
 
-        public const string Version = "1.5.0.0";
+        public const string Version = "1.5.1.0";
         public const string ProductVersion = "5.12.1.0";
     }
 }