Remove remarks about out of support .NET versions (System.R*, System.S*, System.T*)

[gewarren]

Contributes to #12648.

[Copilot]

Pull request overview

This PR updates .NET API reference XML docs to remove or generalize remarks that reference out-of-support .NET (Core/5/6/7) versions, aligning the content with the goal in #12648.

Changes:

• Removed version-qualified remarks and notes across multiple APIs (threading, crypto, interop, reflection, text/JSON).
• Simplified framework/version language in tables and exception descriptions.
• Adjusted some wording to be version-neutral (for example, “starting in .NET X” → unconditional statements).

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
xml/System.Threading/ReaderWriterLockSlim.xml	Simplifies NOTE content about lock behavior.
xml/System.Threading.Tasks/Task.xml	Updates exception documentation to remove version-dependent limits.
xml/System.Text/Encoding.xml	Collapses encoding support table columns and updates UTF-7 note text.
xml/System.Text.Json/Utf8JsonWriter.xml	Removes framework/version-specific numeric formatting notes.
xml/System.Text.Json/Utf8JsonReader.xml	Removes version-specific performance guidance.
xml/System.Text.Json.Serialization/JsonIncludeAttribute.xml	Removes .NET 6/7 limitation note.
xml/System.Security.Cryptography/TripleDESCng.xml	Removes version prefix from exception descriptions.
xml/System.Security.Cryptography/Oid.xml	Removes version qualifiers around property semantics/exceptions.
xml/System.Security.Cryptography/FromBase64Transform.xml	Removes version-dependent behavior explanation for multi-block transform support.
xml/System.Security.Cryptography/CryptoStream.xml	Removes “Starting in .NET 6” qualifier from partial-read behavior notes.
xml/System.Security.Cryptography/AesCng.xml	Removes version prefix from exception descriptions.
xml/System.Security.Authentication/SslProtocols.xml	Removes “starting in .NET 7” phrasing from obsoletion summaries.
xml/System.Runtime.Loader/AssemblyLoadContext.xml	Removes .NET Core 2.0–2.2-specific guidance.
xml/System.Runtime.InteropServices/RuntimeInformation.xml	Rewords emulation behavior note to be version-neutral.
xml/System.Runtime.InteropServices/NativeLibrary.xml	Removes “Starting with .NET 5” qualifier in resolver behavior remark.
xml/System.Runtime.InteropServices/Marshal.xml	Removes version qualifiers around NativeMemory and last-error guidance.
xml/System.Runtime.InteropServices/DllImportAttribute.xml	Simplifies last-error retrieval guidance to version-neutral wording.
xml/System.Reflection/IReflect.xml	Adjusts version ranges described for COM casting behavior.
xml/System.Reflection/Assembly.xml	Removes/relaxes version qualifiers for bundled assembly exception and obsoletion remark.

[wadepickett]

@gewarren , approved assuming current suggestions fixed. I only found one tiny nit beyond what Copilot already suggested.