We currently force Preview as the lang version in our Prerequisites package/dependency. This is now unnecessary since C# 9.0 is stable now.

But we now need to detect the whole range of valid values for the LangVersion property that users might configure, as documented in https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/langversion-compiler-option.

We should check for C# 9.0+ explicitly (which includes latest, latestmajor and preview values) instead.