Drop .NET Standard 2.1 ?

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The reason why you would multi-target .NET Standard 2.0 + .NET Standard 2.1 is to take advantage of APIs in .NET Standard 2.1 that are not available in .NET Standard 2.0. This is not the case for this project where all APIs used are compatible with .NET Standard 2.0. Adding a .NET Standard 2.1 target framework only makes the NuGet package grow in size without any other added benefit.