compatible of .NETStandard 2.1

[yts233]

I opened a solution contains net5.0 and netstandard2.1 projects.
Well, the IDE was stuck on restoring Nuget packages stage:
https://paste.ubuntu.com/p/xGkTQqcNHJ/

And I tried to change the target famework into netcoreapp3.1, but failed again:
https://paste.ubuntu.com/p/H2THHHQxZ3/

In Chinese, I want to say: 太草了