msbuild default tfm

<?xml version="1.0" encoding="utf-8"?>
<Project>
  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;netcoreapp3.0</TargetFrameworks>
    <DefaultTargetFramework Condition="$(TargetFrameworks.Contains(';'))">$(TargetFrameworks.Substring(0, $(TargetFrameworks.IndexOf(';'))))</DefaultTargetFramework>
  </PropertyGroup>
</Project>