Only Support Officially Supported .NET Versions

[warappa]

To simplify the development, this project will switch to supporting only officially supported .NET versions.
.NET Core 3.1 and .NET 5 are out of support for some time now and it's advised anyway to update to supported .NET versions.

As of today this means this library only supports

• .NET 6
• .NET 7
• .NET 8

As an extra, I leave .NET Standard 2.0 where it makes sense.