Remove .NET Framework remarks (System.Text)

[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 removes all remarks from obsolete APIs.

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[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-encoding

[Copilot]

Pull request overview

This PR updates the ECMAXML API reference docs to remove or modernize .NET Framework-specific remarks, links, and behavioral notes, primarily within System.Text (and a couple related namespaces), now that .NET Framework API reference content lives in a separate repo.

Changes:

• Rewrites/removes .NET Framework-specific remarks, notes, and examples (including version-specific behavior callouts).
• Updates article link text from “in the .NET Framework” to “in .NET” and adjusts wording to be implementation-agnostic.
• Normalizes/cleans up some markdown formatting (tables, blank lines, notes).

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
xml/System.Text/UTF8Encoding.xml	Updates encoding remarks/link text and removes .NET Framework version-specific serialization note wording.
xml/System.Text/UTF7Encoding.xml	Updates encoding remarks/link text and removes .NET Framework version-specific serialization note wording.
xml/System.Text/UnicodeEncoding.xml	Cleans up markdown spacing and modernizes encoding article link text.
xml/System.Text/StringBuilder.xml	Removes .NET Core/.NET Framework split language in usage notes and modernizes formatting/composite-formatting wording.
xml/System.Text/EncodingProvider.xml	Rewords remarks to modern .NET framing and updates fallback/encoding-provider guidance.
xml/System.Text/EncodingInfo.xml	Updates example wording to refer to .NET rather than .NET Framework.
xml/System.Text/Encoding.xml	Removes .NET Framework-specific behavioral notes and simplifies/modernizes guidance around encodings and providers.
xml/System.Text/EncoderReplacementFallback.xml	Modernizes wording from “.NET Framework classes” to “.NET classes”.
xml/System.Text/EncoderFallbackBuffer.xml	Modernizes wording and updates related article title text.
xml/System.Text/EncoderFallback.xml	Modernizes wording and updates related article title text.
xml/System.Text/EncoderExceptionFallback.xml	Modernizes wording from “.NET Framework” to “.NET”.
xml/System.Text/DecoderReplacementFallback.xml	Modernizes wording from “.NET Framework classes” to “.NET classes”.
xml/System.Text/DecoderFallbackBuffer.xml	Modernizes wording and updates related article title text.
xml/System.Text/DecoderFallback.xml	Modernizes wording and updates related article title text.
xml/System.Text/DecoderExceptionFallback.xml	Modernizes wording from “.NET Framework” to “.NET”.
xml/System.Text/CodePagesEncodingProvider.xml	Modernizes wording and reworks guidance to avoid .NET Core-specific framing while keeping provider usage instructions.
xml/System.Text.RegularExpressions/Regex.xml	Updates remarks language around CompileToAssembly to remove .NET Framework-specific phrasing.
xml/System.Text.Json/JsonElement.xml	Removes .NET Core callouts in remarks and adjusts whitespace/formatting.