Update library guidance

[Jericho]

In particular, the "Cross-platform targeting" section needs to be refreshed, I think.

• What are the latest recommendations for library authors?
• What are the most "reasonable" frameworks we should target if we want our library to be used by the largest audience? I realize this might be somewhat subjective but let's assume library authors want to reach developers who use "reasonably" up-to-date frameworks (in other words, let's ignore net45, netstandard1.x, etc.). For example, is net48;netstandard2.1;net5.0;net6.0 reasonable?
• Does it make sense to continue targeting netstandard at all?
• Is it redundant to target both net5 and net6? If so, I assume it makes more sense to only target net6, right?
• How will these recommendations change when net7 is released?

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Document Details

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• ID: 5b3638c9-ea3c-1c9e-2763-5c3e8e965663
• Version Independent ID: aff99d6b-9471-9277-160b-670ee8028b53
• Content: Cross-platform targeting for .NET libraries
• Content Source: docs/standard/library-guidance/cross-platform-targeting.md
• Product: dotnet-fundamentals
• GitHub Login: @JamesNK
• Microsoft Alias: jamesnk

[ArthurHNL]

I would like to point out that .NET 5 is out of support by now. So this article is de-facto recommending to target a framework that is out of support, as it suggests adding a reference to net5.0 or later.

[Jericho]

@gewarren I noticed that you are preparing a list of documents to update for .NET 8. Maybe the guidelines for library authors should also be on that list?