Obsolete AuthenticationManager of System.Net.Requests

[deeprobin]

Fixes #77459. This PR obsoletes the class AuthenticationManager.

Changed API Scheme

namespace System.Net;

+ [System.ObsoleteAttribute(
+   "AuthenticationManager is not supported. Methods will no-op or throw PlatformNotSupportedException.",
+    DiagnosticId = "SYSLIB0009",
+    UrlFormat = "https://aka.ms/dotnet-warnings/{0}")]
public partial class AuthenticationManager
{
    // ...
}

Changes made

• Change message of existing diagnostic SYSLIB0009
• Apply SYSLIB0009 obsoletion at class-level (AuthenticationManager)
• Disable diagnostic in AuthenticationManager tests
• Update runtime docs

/cc @MihaZupan

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

---

Fixes #77459. This PR obsoletes the class AuthenticationManager.

Changed API Scheme

namespace System.Net;

+ [System.ObsoleteAttribute(
+   "AuthenticationManager is not supported. Methods will no-op or throw PlatformNotSupportedException.",
+    DiagnosticId = "SYSLIB0009",
+    UrlFormat = "https://aka.ms/dotnet-warnings/{0}")]
public partial class AuthenticationManager
{
    // ...
}

Changes made

• Change message of existing diagnostic SYSLIB0009
• Apply SYSLIB0009 obsoletion at class-level (AuthenticationManager)
• Disable diagnostic in AuthenticationManager tests
• Update runtime docs

/cc @MihaZupan

Author:	deeprobin
Assignees:	-
Labels:	area-System.Net, new-api-needs-documentation
Milestone:	-

[rzikm]

LGTM

[MihaZupan]

Thank you

[wfurt]

It seems like test failures are relevant. We will need to work out suppression as the change is intended.