Update guidance for targeting ASP.NET Core from libraries for .NET 6

This doc has a bunch of content in it that is specific to .NET Core 3.0 or 3.1.

For Blazor libraries, we now recommend that you target .NET 5. If you need to also support .NET Core 3.1 LTS, then you should target .NET Core 3.1. You can optionally multi-target netcoreapp3.1 and net5.0 if you want to take advantage of new features in .NET 5. Targeting .NET Standard should no longer be necessary now that Blazor WebAssembly 3.2 is out of support. Setting the RazorLangVersion should no longer be necessary if you're not targeting .NET Standard.

@pranavkm

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#### Document Details

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* ID: 98629146-7d64-9f48-6e45-51bafe8c7116
* Version Independent ID: cdb2b89f-bf1c-f313-0062-7f6b7ef8b611
* Content: [Use ASP.NET Core APIs in a class library](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/target-aspnetcore?view=aspnetcore-5.0&tabs=visual-studio)
* Content Source: [aspnetcore/fundamentals/target-aspnetcore.md](https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/fundamentals/target-aspnetcore.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-fundamentals**
* GitHub Login: @scottaddie
* Microsoft Alias: **scaddie**