Prepare .NET libraries for trimming: Add multi-targeting H2 with our recommendations

[Rick-Anderson]

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Finally, recently we've learned that multi-targeting is not only very common, but basically a must have when enabling trimming. (This is based on experience trying to make some NuGets trim compatible). I think the document should have a section about the multi-targeting and our recommendations around it. But the core point is that very few libraries will only target latest TFM. By far the most common case is either library targets netstandard2.0 or it multi-targets net472, netstandard2.0 and net6.0 (or some combination of these).

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• ID: f3d1f980-933d-a2a8-1645-098c2a4bc2a2
• Version Independent ID: 80dc5a35-aee1-fa68-5a21-78c628e9ff41
• Content: Prepare .NET libraries for trimming - .NET
• Content Source: docs/core/deploying/trimming/prepare-libraries-for-trimming.md
• Product: dotnet-fundamentals
• GitHub Login: @sbomer
• Microsoft Alias: svbomer

[sbomer]

Leaving a note here to mention the new behavior in the <IsTrimmable>true</IsTrimmable> part of https://learn.microsoft.com/dotnet/core/deploying/trimming/prepare-libraries-for-trimming.

[sbomer]

I'm adding a breaking change notice that mentions multi-targeting: #36775. We still need a dedicated section in the general trimming docs though. When that's added, we should also link to it from the breaking change notice.