Dropping support for .NET Standard 2.1

[CXuesong]

WCL users,

As this project enters its 7th year, there has been many changes -- while WCL is still in its v0.8.0-int versions 🌚

Unfortunately I've been busy throughout the last year and there aren't any activities in this repo. I've try to review your PRs in the coming months and there are still some plans for the future.

• Dropping support on .NET Standard 2.1 (covered in this discussion)
  • This means all the WCL libraries will target .NET 6.0 / 8.0 instead.
• Switching from Newtonsoft.Json to System.Text.Json (some huge work to do huh)
  • This won't happen in the near future, though.

While there is a statement that ".NET Standard is just a set of specifications and they won't get outdated" in dotnet/standard#1681, .NET Standard 2.1 is de-facto the last version of .NET Standard and its minimum .NET Core implementation version, 3.0 & 3.1 (LTS) has already reached their EOL. Personally I'd like to embrace the latest technologies and introduce some fancy record classes to the project. Now the compatibility for .NET Standard 2.1 (.NET Core 3.1) becomes a major blocker because I don't want to introduce too many #ifs.

Same as mentioned in #107, this is a breaking change, so I'll release the next version (after targeting platform change) as v0.8.0 or v1.0.0. You can still authoring your applications on older versions of WCL, but you won't be able to receive WCL feature updates on those platforms not supported by .NET 6.0

If you have any comments regarding to this proposal, please let me know. Thank you!