Avoid `file private` visibility when targeting LangVersion < 11

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.

Locate the RequestDelegateGenerator and inspect where it emits the generated class visibility and handles the target language version. Verify the behavior for language versions below C# 11 and newer versions; done means older targets compile with internal visibility while newer targets retain file private visibility.

Written by the indexing model from the issue text.