.NET Standard 2.1 for UWP and Unity

Those two are still TBD. Please update since there are many libraries targeting .NET 2.1 and are therefore unavailable for UWP and Unity. Thanks!

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#### Document Details

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* ID: 9eb8b774-c6f0-8235-6964-15219baf2ea7
* Version Independent ID: 6819a4a7-7527-d8c7-e132-767f7344e00e
* Content: [.NET Standard](https://docs.microsoft.com/en-us/dotnet/standard/net-standard#net-implementation-support)
* Content Source: [docs/standard/net-standard.md](https://github.com/dotnet/docs/blob/main/docs/standard/net-standard.md)
* Product: **dotnet**
* Technology: **dotnet-standard**
* GitHub Login: @gewarren
* Microsoft Alias: **gewarren**