runtime: ConfigurationManager is not trim compatible

Using the latest 6.0 SDK

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="System.Configuration.ConfigurationManager" Version="6.0.0-preview.3.21153.4" />
  </ItemGroup>

</Project>

using System;
using System.Configuration;

namespace ConsoleApp7
{
    class Program
    {
        static void Main(string[] args)
        {
            string sAttr = ConfigurationManager.AppSettings.Get("Key0");
            Console.WriteLine("hello" + sAttr);
        }
    }
}

❯ dotnet publish -c Release -r win-x64 -p:PublishTrimmed=true -p:TrimMode=link
❯ C:\Users\eerhardt\source\repos\ConsoleApp7\ConsoleApp7\bin\Release\net6.0\win-x64\publish\ConsoleApp7.exe
Unhandled exception. System.Configuration.ConfigurationErrorsException: Configuration system failed to initialize
 ---> System.MissingMethodException: Cannot dynamically create an instance of type 'System.Configuration.ClientConfigurationHost'. Reason: No parameterless constructor defined.
   at System.RuntimeType.ActivatorCache..ctor(RuntimeType rt)
   at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean wrapExceptions)
   at System.Activator.CreateInstance(Type type, Boolean nonPublic, Boolean wrapExceptions)
   at System.Activator.CreateInstance(Type type, Boolean nonPublic)
   at System.Configuration.TypeUtil.CreateInstance(Type type)
   at System.Configuration.Internal.ConfigSystem.System.Configuration.Internal.IConfigSystem.Init(Type typeConfigHost, Object[] hostInitParams)
   at System.Configuration.ClientConfigurationSystem..ctor()
   at System.Configuration.ConfigurationManager.EnsureConfigurationSystem()
   --- End of inner exception stack trace ---
   at System.Configuration.ConfigurationManager.EnsureConfigurationSystem()
   at System.Configuration.ConfigurationManager.PrepareConfigSystem()
   at System.Configuration.ConfigurationManager.GetSection(String sectionName)
   at System.Configuration.ConfigurationManager.get_AppSettings()
   at ConsoleApp7.Program.Main(String[] args) in C:\Users\eerhardt\source\repos\ConsoleApp7\ConsoleApp7\Program.cs:line 12

---

Because https://github.com/dotnet/runtime/pull/48428 marked all assemblies building from dotnet/runtime as IsTrimmable=true, System.Configuration.ConfigurationManager is now getting aggressively trimmed. However, since we haven’t addressed the ILLink warnings in the assembly, it is not trim compatible.

We should probably remove IsTrimmable from this assembly until we have addressed its warnings.

cc @marek-safar @sbomer