Use new NTAuth corefx API

[Tratcher]

Today NegotiateAuth is implemented using reflection:
https://github.com/aspnet/AspNetCore/blob/b6d8c96a98c9e5673cd10162616ed98799c60d88/src/Security/Authentication/Negotiate/src/Internal/ReflectedNegotiateState.cs

https://github.com/dotnet/corefx/issues/36896 adds a new public API for Negotiate/NTLM auth. When that's ready (post 3.1), switch to the public API.

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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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[Tratcher]

Fixed by #42364