Remove remarks about out of support .NET versions (System namespace)

[gewarren]

Contributes to #12648.

[Copilot]

Pull request overview

This PR updates System namespace reference docs to remove or generalize remarks that call out out-of-support .NET/.NET Core versions, aligning with issue #12648.

Changes:

• Rephrases several remarks to remove explicit references to older .NET/.NET Core versions.
• Simplifies or removes version-specific behavioral notes in various System API topics.
• Updates a few links / phrasing to be version-agnostic (for example, tutorial titles and project wording).

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
xml/System/Uri.xml	Rewords Unix implicit file path remark to remove “.NET Core” mention.
xml/System/UIntPtr.xml	Removes explicit .NET 7 targeting qualifiers from remarks about interfaces / C# behavior.
xml/System/Type.xml	Rephrases reflection-ordering remarks to remove older-version comparisons.
xml/System/TimeZoneInfo.xml	Removes older-version qualifiers around IANA support and caching behavior; refines ICU-related remarks.
xml/System/String.xml	Removes a table mapping Unicode versions to older .NET releases; rewords a sample-project reference.
xml/System/MarshalByRefObject.xml	Generalizes obsolescence remarks without referencing .NET 5.
xml/System/IntPtr.xml	Removes explicit .NET 7 targeting qualifiers from remarks about interfaces / C# behavior.
xml/System/IAsyncDisposable.xml	Updates tutorial link text to remove older-version naming.
xml/System/Guid.xml	Removes “starting with .NET 6” qualifier from non-Windows entropy note.
xml/System/GC.xml	Removes version-specific remark in AllocateArray/AllocateUninitializedArray docs (currently replaced with a placeholder in one spot).
xml/System/Environment.xml	Removes “.NET 5 and later” / “starting with .NET 6” qualifiers from remarks.
xml/System/Char.xml	Removes “introduced in .NET Core 3.0” qualifier for System.Text.Rune.