Update diagnostic docs for SYSLIB0009

[deeprobin]

Summary

Follow-up for dotnet/runtime#93171.
Change obsoletion documentation: Whole class marked as obsolete.

See dotnet/runtime#77459.

/cc @MihaZupan

---

Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/9.0.md	Breaking changes in .NET 9
docs/core/compatibility/core-libraries/9.0/obsolete-apis-with-custom-diagnostics.md	API obsoletions with non-default diagnostic IDs (.NET 9)
docs/fundamentals/syslib-diagnostics/obsoletions-overview.md	Obsolete features in .NET 5+
docs/fundamentals/syslib-diagnostics/syslib0009.md	SYSLIB0009: AuthenticationManager is not supported

[MihaZupan]

Thank you

[gewarren]

Thank you. Since this is for .NET 9, we shouldn't merge it yet.

[gewarren]

We also need to:

• Update https://github.com/dotnet/docs/blob/main/docs/fundamentals/syslib-diagnostics/obsoletions-overview.md
• Create a new article like this but for .NET 9: https://github.com/dotnet/docs/blob/main/docs/core/compatibility/core-libraries/8.0/obsolete-apis-with-custom-diagnostics.md

[wfurt]

can this be merged @gewarren?

[gewarren]

@wfurt It can be merged if we change the date that's causing a warning, but why publish .NET 9 documentation now when the first preview won't be available until February?

[wfurt]

so can complete runtime issue tracking it :)