unlock LangVersion latest for all c# snippets

[SimonCropp]

so that we can modernize samples

i made a change to one sample to prove the approach works

[BillWagner]

Hi @SimonCropp

I like the direction here to make it easier for more of our code snippets to use the latest language features. Our team has been updating snippets as we make changes to them.

However, we had a couple concerns:

1. This pulls all the snippets toward an unsupported scenario: using a higher C# language with a lower TFM. Users could accidentally fall into an unsupported scenario. Or, if they copied a single project rather than clone the repo, the samples might not build.
2. Some snippets are set to an older version by design. This is either because it's the starting point for a migration or a snippet that targets .NET Framework (WebForms, for example). It feels odd to add back the default setting.

So, we wonder about a different way to achieve the same goal. Suppose the Directory.build.props file set the TFM. Something like:

<?xml version="1.0" encoding="utf-8"?>
<Project>
  <PropertyGroup>
    <TargetFramework Condition="$(MSBuildProjectExtension) == '.csproj'">net10.0</TargetFramework>
  </PropertyGroup>
</Project>

(Or start with net9.0 for now).

Then, as we make any updates, we remove the TargetFramework node for the project. In time, the only projects with a TFM either need some extensions, are designed to be an older framework and version, or are using the current preview version.

What are your thoughts?