Update cross-platform targeting guidance to prioritize modern .NET over netstandard2.0

[Copilot]

This PR updates the cross-platform targeting guidance to reflect current best practices by recommending modern .NET versions (net6.0+) as the starting point for new libraries instead of netstandard2.0.

Background

The current documentation suggests starting with netstandard2.0 as the default for cross-platform targeting. However, as noted in the issue, this advice is outdated and may inadvertently:

1. Discourage the use of modern .NET idioms
2. Block adoption of AOT/trimming related technologies
3. Confuse cross-platform compatibility needs with breadth targeting requirements

Changes Made

Primary recommendation updated: Changed from "DO start with including a netstandard2.0 target" to "DO start with including a net6.0 target or later for new libraries."

Added conceptual clarity: Introduced a distinction between:

• Cross-platform compatibility: Working across different operating systems (Windows, Linux, macOS)
• Breadth targeting: Supporting a wide range of .NET implementations and versions

Repositioned netstandard2.0: Changed to "CONSIDER including a netstandard2.0 target if you need broad compatibility or .NET Framework support" - positioning it as a specific-use case rather than the default.

Updated examples: The multi-targeting project file example now shows net6.0;netstandard2.0 instead of netstandard2.0;net462 to reflect modern practices.

Impact

The updated guidance encourages developers to:

• Start with modern .NET versions that provide access to latest APIs and performance improvements
• Enable features like AOT and trimming by default
• Only target netstandard2.0 when there's a specific need for .NET Framework support or maximum compatibility

This aligns with the ecosystem-wide push toward modern .NET while still providing clear guidance on when older targets are appropriate.

Fixes #47458.

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Internal previews

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

[baronfel]

Suggested change

	✔️ DO start with including a `net6.0` target or later for new libraries.
	✔️ DO start with including a `net8.0` target or later for new libraries.

let's use the oldest in support version as a baseline here.