DateOnly in Netstandard2.1

[emitrackman]

Describe the bug

Generated code contains references to DateOnly which is not supported by netstandard2.1 and thus can't be used in Unity
Compilation fails with
ZeroQL/config.zeroql.json.g.cs(11736,404): error CS0246: The type or namespace name 'DateOnly' could not be found (are you missing a using directive or an assembly reference?)

How to Reproduce

Follow instructions from here:
https://github.com/byme8/ZeroQL/wiki/Unity-setup

Expected behavior

It compiles.

The GraphqQL schema that fails

Any schema that has Date in it will do.

Environment (please complete the following information):

• Nuget package version [6.2.1]
• IDE: none, CLI
• .Net Version : [NetStandard2.1, Unity]

Additional information

I noticed before ZeroQL was using DateTime so it's probably this change broke it
bc83141

maybe we can shield it with #if !NETSTANDARD and use DateTime instead of DateOnly there.