Fix nuget package to not include stellar-dotnet-sdk and stellar-dotnetstandard-sdk at the same time

[ORuban]

Problem Nuget package for .NETStandard contains both stellar-dotnet-sdk and stellar-dotnetstandard-sdk assemblies and so compiler doesn't know what to use.

To Reproduce

• Create a netstandard2.0 project
• Add nuget package stellar-dotnet-sdk 2.1.0-beta2
• Try to use any type from stellar_dotnet_sdk namespace

After that, build fails with the following error: The type '<TypeName>' exists in both 'stellar-dotnet-sdk, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null' and 'stellar-dotnetstandard-sdk, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null'

[elucidsoft]

A new nuget package has been release 2.1-beta3 that resolves this...