[FEATURE REQUEST]: Stop targeting .net standard (both 2.0 and 2.1)

**Is your feature request related to a problem? Please describe.**

For simplicity, does it make sense to **stop** targeting .net standard and to **only** target .net 8?
I don't think other github projects will consume from this one very often. I don't think we gain anything from targeting .net standard. I think it is far more common to target .net 6 and .net 8, without the .net standard cruft.

**Describe the solution you'd like**

With the next round of project file changes, I propose targeting .net 8 and nothing else.

**Describe alternatives you've considered**

In the very least we should remove .net standard 2.1 and only target .net standard 2.0.
I think .net standard 2.1 was an odd duck. It is an overkill and unnecessary here. Also I don't think it is supported by the MessagePack serialization dependendy.

**Additional context**

I was in the process of trying to migrate to .net 8 with MessagePack and I noticed the unusual platform targeting capabilities in this community. I think the capabilities are somewhat dated now that .net 8 is LTS. I think we would benefit from **simplifying** our targeting of platforms.