Add netstandard2.0 tfm to Spectre.Console.Cli.Testing

Author: patriksvensson

src/Spectre.Console.Cli.Testing/Spectre.Console.Cli.Testing.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-      <TargetFrameworks>net10.0;net9.0;net8.0</TargetFrameworks>
+      <TargetFrameworks>net10.0;net9.0;net8.0;netstandard2.0</TargetFrameworks>
       <IsPackable>true</IsPackable>
       <IsAotCompatible Condition="'$(TargetFramework)' != 'netstandard2.0'">false</IsAotCompatible>
       <IsTrimmable>false</IsTrimmable>