Assemblies should target netstandard2.2 for greater compatibility

[ElanHasson]

Ray/src/Directory.Build.props

Line 3 in cf42e6b

<TargetFrameworks>netcoreapp2.2;netcoreapp3.1</TargetFrameworks>

The libraries should target netstandard2.0 to increase application target runtimes.

[newbe36524]

Cause there are some class include in netstandard2.1/etcoreapp2.2/netcoreapp3.1 but not netstandard2.0 ,Such as Memory or somthing for performance.

[ElanHasson]

https://github.com/RayTale/Ray/blob/master/Test/Directory.Build.props#L3

Why dual targeting here? are people using 2.1 and 3.1?

[ElanHasson]

I just validated netstandard2.1 works fine.

netstandard2.0 doesn't have span and a few other things.

If we can drop netcoreapp2.2 we can use netstandard2.1 .

Let me know if you'd like a pull request for this.

[ElanHasson]

Feel free to reject the PR if you need to keep netcoreapp2.2. That is the only reason we can't move forward.