Stop using init-only setter for netstandard

[zhiyuanliang-ms]

Why this PR?

#435

Visible change

Replace init setter to set

[zhenlan]

Is it possible we can add test to make sure we are netstandard/netfx 4.x compatible?

[zhiyuanliang-ms]

Is it possible we can add test to make sure we are netstandard/netfx 4.x compatible?

Currently, the TargetFrameworks is netstand2.0 but the LangVersion is 9.0, maybe we should set the LangVersion to 7.3 or remove the LangVersion.
https://github.com/microsoft/FeatureManagement-Dotnet/blob/main/src/Microsoft.FeatureManagement/Microsoft.FeatureManagement.csproj

I checked other .net libraries which targets on netstandard, I found they didn't specify the LangVersion
https://github.com/dotnet/runtime/blob/main/src/libraries/Microsoft.Bcl.TimeProvider/src/Microsoft.Bcl.TimeProvider.csproj

Maybe, we should do the same thing.

[zhiyuanliang-ms]

@jimmyca15 @zhenlan

Maybe we should set the LangVersion to 8.0. The things get quite tricky here.
We have an API: IAsyncEnumerable<string> GetFeatureNamesAsync() of IFeatureManager,

However, async stream is not available in C# 7.3 (need 8.0+), so the GetFeatureNamesAsync will not work for .NET framework app.

[alexaka1]

The await foreach() syntax is exclusive to 8.0 and up, but you can still use the type manually in 7.3, getting an enumerator and calling next asynchronously in a while loop.
https://learn.microsoft.com/en-us/archive/msdn-magazine/2019/november/csharp-iterating-with-async-enumerables-in-csharp-8#a-tour-through-async-enumerables
https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iasyncenumerable-1.getasyncenumerator?view=netframework-4.8#system-collections-generic-iasyncenumerable-1-getasyncenumerator(system-threading-cancellationtoken)

[jimmyca15]

Using language version 8.0.0 would ensure we couldn't write init only setters. It also prevents us from taking advantage of new language features but that may be what makes sense since we are supporting netstandard.

[jimmyca15]

We should mention why we don't use later than version 8.0

[zhiyuanliang-ms]

/AzurePipeline run

[zhiyuanliang-ms]

/AzurePipeline run

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