dotnet-svcutil.xmlserializer error with targetframework netstandard2.1

### Description
I followed the readme on [here](https://github.com/dotnet/core/blob/master/samples/dotnet-svcutil.xmlserializer-instructions.md) which is slightly outdated as the current version of the dotnet-svcutil.xmlserializer is 1.2.0 and and 4.8.1.

However, when I try building the csproj file in both in VS and CLI (dotnet build) I'm getting compiler error:

> C:\Users\xxxxx\\.nuget\packages\dotnet-svcutil.xmlserializer\1.2.0\build\dotnet-svcutil.xmlserializer.targets(20,7): error MSB4064: The "TargetFrameworkMoniker" parameter is not supported by the "ResolvePackageAssets" task. Verify the parameter exists on the task, and it is a settable public instance property.
> C:\Users\xxxxx\\.nuget\packages\dotnet-svcutil.xmlserializer\1.2.0\build\dotnet-svcutil.xmlserializer.targets(16,5): error MSB4063: The "ResolvePackageAssets" task could not be initialized with its input parameters.

### Configuration

.NET SDK 5.0.103 Installed
Main Application (AspnetCore) targeting NET Core 3.1
dotnet-svcutil.xmlserializer to a project file is targeting netstandard 2.1
OS: Windows 10