Target newer .NET versions

[bettinaheim]

It would be nice to support newer TargetFrameworks. Using it in a netcoreapp3.1 project causes a bunch of

warning MSB3277: Found conflicts between different versions of "Microsoft.Win32.Registry" that could not be resolved.

Project file:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="AssemblyInfo.fs" />
    <Compile Include="DesignTimeBuild.fs" />
    <Compile Include="CommandLineArguments.fs" />
    <Compile Include="Program.fs" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="CommandLineParser" Version="2.8.0" />
    <PackageReference Include="Microsoft.Build" Version="16.9.0" ExcludeAssets="runtime" />
    <PackageReference Include="Microsoft.Build.Framework" Version="16.9.0" ExcludeAssets="runtime" />
    <PackageReference Include="Microsoft.Build.Locator" Version="1.4.1" />
    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="16.9.0" ExcludeAssets="runtime" />
  </ItemGroup>
</Project>

See also https://github.com/FluentValidation/FluentValidation/issues/638.