update compile condition

[WeihanLi]

Since we're targeting for netstandard2.0/net8.0, update to #if NET to avoid changes in future target framework updates

[rabbitism]

Enum.GetValuesAsUnderlyingType only works for 7+. Is this feasible?

[WeihanLi]

NET is only defined for net5.0+, not defined for netstandard2.0, see details here: https://learn.microsoft.com/en-us/dotnet/standard/frameworks#preprocessor-symbols

When we pack the package, there're two library/dll generated for netstandard2.0 and net8.0, this can be found from the produced nupkg file, and net8.0 dll would only be used when the project is using net8.0+

And when we decompile the netstandard2.0 dll, it would still behave like before

same as before in net8.0 dll