Target framework .net 2.0 too matching with System.CommmandLine

Author: jbtule

src/FSharp.SystemCommandLine/FSharp.SystemCommandLine.fsproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
 	<Version>0.15.0-beta4</Version>
 	<Description>F# computation expressions for working with the System.CommandLine API.</Description>