Remove .NET Framework remarks (System.Net/*)

[gewarren]

.NET Framework API ref has moved to its own repo (https://github.com/dotnet/dotnetfw-api-docs), so we can clean up .NET Framework remarks, exceptions, and code examples out of this repo. Contributes to #12513.

Removes remarks and examples related to:

• .NET Framework versions
• Code-access security
• Configuring apps via app.config file
• App domains

Also remarks all remarks from obsolete APIs.

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Tagging subscribers to this area: @dotnet/ncl

[Copilot]

Pull request overview

This PR updates the System.Net API reference XML to remove .NET Framework-specific remarks, links, and guidance (for example, CAS, app.config, and application domain notes) now that .NET Framework reference docs live in a separate repo.

Changes:

• Removed or simplified .NET Framework-targeted remarks, examples, and cross-links across System.Net types.
• Updated various “network tracing” notes to remove .NET Framework article links and other framework-specific phrasing.
• Simplified/removed documentation for obsolete APIs and some legacy behaviors.

Reviewed changes

Copilot reviewed 31 out of 33 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
xml/System.Net/WriteStreamClosedEventArgs.xml	Updates Obsolete attribute text (non-Docs metadata) and leaves placeholders in Docs.
xml/System.Net/WebResponse.xml	Removes .NET Framework-specific notes; introduces placeholder remarks.
xml/System.Net/WebRequest.xml	Removes .NET Framework-specific serialization remark/link; introduces placeholder remarks.
xml/System.Net/WebProxy.xml	Updates “not supported” note/exception text; note wording now mismatches API kind.
xml/System.Net/WebPermissionAttribute.xml	Removes CAS/declarative security guidance; replaces multiple remarks with placeholders.
xml/System.Net/WebPermission.xml	Removes CAS guidance; replaces many member remarks with placeholders.
xml/System.Net/WebHeaderCollection.xml	Removes .NET Framework vs .NET Core header restrictions/length-validation notes and exceptions.
xml/System.Net/WebException.xml	Removes Windows Store/Windows 8.x-specific warning.
xml/System.Net/SocketPermissionAttribute.xml	Removes CAS/declarative security guidance; replaces multiple remarks with placeholders.
xml/System.Net/SocketPermission.xml	Removes CAS guidance; replaces many member remarks with placeholders.
xml/System.Net/ServicePointManager.xml	Removes .NET Framework version-specific text/links; rewrites TLS opt-out guidance; adds default-value statement.
xml/System.Net/ServicePoint.xml	Removes extensive remarks/examples and several .NET Framework-only notes; keeps obsolete include blocks.
xml/System.Net/SecurityProtocolType.xml	Removes “.NET Framework” wording and related article link.
xml/System.Net/IWebProxyScript.xml	Removes app-domain/PAC/WPAD discussion; replaces with placeholder remark.
xml/System.Net/ICredentialPolicy.xml	Removes “application domain” phrasing and minor formatting changes.
xml/System.Net/HttpWebResponse.xml	Removes .NET Framework-only tracing links; adjusts exception wording; introduces a markdown formatting issue.
xml/System.Net/HttpWebRequest.xml	Removes large amounts of remarks/examples/links and some framework-specific guidance; replaces multiple remarks with placeholders.
xml/System.Net/HttpListenerResponse.xml	Removes version-specific header-length exception; adjusts exception formatting.
xml/System.Net/HttpListenerRequest.xml	Removes tracing link and network settings schema links.
xml/System.Net/HttpListenerPrefixCollection.xml	Removes tracing link text in usage blocks.
xml/System.Net/HttpListenerContext.xml	Removes tracing link text in usage blocks.
xml/System.Net/HttpListener.xml	Removes tracing link text in usage blocks; retains older OS PNSE text.
xml/System.Net/GlobalProxySelection.xml	Removes .NET Framework configuration article link.
xml/System.Net/FtpWebResponse.xml	Removes .NET Framework tracing link and framework-version remark; introduces placeholder remark.
xml/System.Net/FtpWebRequest.xml	Removes .NET Framework mentions; changes tracing notes; documents Proxy as unsupported and removes related altmembers/examples.
xml/System.Net/FileWebResponse.xml	Removes CAS mention and framework-version remark; introduces placeholder remark.
xml/System.Net/FileWebRequest.xml	Removes CAS mention; simplifies tracing note and whitespace.
xml/System.Net/DnsPermissionAttribute.xml	Removes CAS/declarative security guidance and altmember; introduces placeholder remarks.
xml/System.Net/DnsPermission.xml	Removes security-zone defaults; replaces many member remarks with placeholders.
xml/System.Net/Dns.xml	Removes .NET Framework tracing links across many members.
xml/System.Net/BindIPEndPoint.xml	Removes “.NET Framework” phrasing from remarks.
xml/System.Net/AuthenticationManager.xml	Removes detailed remarks and parameter exceptions; marks APIs as always PlatformNotSupported with placeholder remarks.

[BillWagner]

This LGTM. Let's