Update Mapster.Core to target netstandard2.0

[ian-g-holm-intel]

I have an application with a plugin interface that lets plugins map custom types. However, these plugins must target netstandard2.0 and cannot target something like .Net 8. I don't see any reason why Mapster.Core couldn't target netstandard2.0 since it is a lightweight library for attributes, enums, etc.

I cloned and built the project with this change and there were no errors. So do you think this is possible? I can make a pull request if necessary.

Thanks.