Update library guidance

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?

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 5b3638c9-ea3c-1c9e-2763-5c3e8e965663
* Version Independent ID: aff99d6b-9471-9277-160b-670ee8028b53
* Content: [Cross-platform targeting for .NET libraries](https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting?source=docs)
* Content Source: [docs/standard/library-guidance/cross-platform-targeting.md](https://github.com/dotnet/docs/blob/main/docs/standard/library-guidance/cross-platform-targeting.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**