Avoid file private visibility when targeting LangVersion < 11

[captainsafia]

The RequestDelegateGenerator currently emits a class with file private visibility to avoid leaking implementation details to user space. This can be problematic if users are targeting a language version below C# 11. To avoid this, we'll want to use internal visibility for those scenarios.

[ghost]

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We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
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[eerhardt]

We should just error out if you aren't using a new enough C#. It is what we do in other source generators - i.e. Regex and Config Binder. ex. dotnet/runtime#83996

[mitchdenny]

Does raise an interesting question. With source generators the language version we rely on when we release 8.0 becomes part of the contract. Moving to newer language features could be considered a breaking change.