cross-platform-targeting: remove Xamarin/UWP-era graphics, modernize content

[Copilot]

The article contained two diagrams showing a pre-.NET-5 ecosystem (Xamarin, .NET Core, UWP, netcoreapp2.0) that no longer reflect current .NET. All UWP references in prose and code were also stale.

Changes

• Deleted both outdated images (platforms-netstandard.png, nuget-package-multiple-assemblies.png)
• Replaced the NuGet multi-targeting diagram with prose describing the netstandard2.0/net8.0 assembly selection behavior
• Code example: WINDOWS_UWP → NET8_0_WINDOWS (a TFM-based preprocessor symbol defined for the net8.0-windows target)

// Before
#elif WINDOWS_UWP
    return CallUwpApi();

// After
#elif NET8_0_WINDOWS
    return CallWindowsApi();

• Prose: "ahead-of-time compilation on iOS or UWP" → "iOS or Android"; "UWP control toolkit" → "WinUI control toolkit"
• Updated ms.date

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Internal previews

📄 File	🔗 Preview link
docs/standard/library-guidance/cross-platform-targeting.md	docs/standard/library-guidance/cross-platform-targeting

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Internal previews

📄 File	🔗 Preview link
docs/standard/library-guidance/cross-platform-targeting.md	docs/standard/library-guidance/cross-platform-targeting

[Copilot]

Pull request overview

This PR modernizes the cross-platform targeting guidance by removing legacy Xamarin/UWP-era diagrams and updating the accompanying prose and sample code to reflect current .NET multi-targeting and Windows platform targeting.

Changes:

• Removed two outdated ecosystem/packaging diagrams from the article.
• Replaced the NuGet multi-targeting diagram with prose describing netstandard2.0 vs net8.0 asset selection.
• Updated UWP references in prose and changed the sample preprocessor symbol from WINDOWS_UWP to WINDOWS.

Reviewed changes

Copilot reviewed 1 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/standard/library-guidance/cross-platform-targeting.md	Removes outdated diagram references, updates UWP-era wording, adds updated multi-targeting explanation, and updates conditional compilation example.
docs/standard/library-guidance/media/cross-platform-targeting/platforms-netstandard.png	Deletes an outdated pre-.NET 5 ecosystem diagram.
docs/standard/library-guidance/media/cross-platform-targeting/nuget-package-multiple-assemblies.png	Deletes an outdated NuGet multi-targeting diagram.