Readd basetargets in Microsoft.ML.OnnxRuntime.csproj

[mszhanyi]

Description

Motivation and Context

Now, the nightly Microsoft.ML.Onnxruntime.Managed Nuget Packag couldn't be added in dotnet console program in VS2022 with target framework .NET 6.0.
I just restore it to previous setting to make it work.

[yuslepukhin]

I do not know what the motivation was for removal of net6.0. Generally, portable libraries are required to implement netstandard2.1.
Such a library should run pretty much anywhere regardless of .net version. What is the specific issue?

[mszhanyi]

I do not know what the motivation was for removal of net6.0. Generally, portable libraries are required to implement netstandard2.1. Such a library should run pretty much anywhere regardless of .net version. What is the specific issue?

Nightly Microsoft.ML.Onnxruntime.Managed Nuget Package couldn't be loaded

Build log says "The target framework 'netcoreapp3.1' is out of support ". So do we really need to add it back?

[yuslepukhin]

They are all old now, but there is not a replacement for NETSTANDARD and NetCore if you want to have a portable library and/or executable that runs on a variety of platforms including Linux, and we need to support it.

Here is the rule. If you want a portable library, one needs NETSTANDARD2.0 compliance. If you want a portable executable, you need NetCore3.1 compliance. This means that our tests need to be compliant.

The reason why our library has NetCore3.1 target may be because some source files are shared between the lib and the tests.
So adding NetCore3.1 automatically alerts the developer who is using IDE if the code he writes is non-compliant.
Thus, I would not remove NetCore3.1.

See this for more info.