I've recomment to support for https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10#file-scoped-namespace-declaration feature. Right now it is like a standard to create all new code outlined.
You no need support all features I suppose (some features really exotic and more like experimental). But major features in each C# lang versions should be present in your parser. Good for you - this features are really not to much. For example, in C# 11 it is only generic attributes and raw literals (not sure this is really required to make some changes in the parser) that I've seen in real apps sources.
So my proposal is - do not waste a time for support all features in news lang versions. Just a really popular and usefull things are really need (by feedbacks you can understand what is popular and not).