Added multitargeting

[nilvon9wo]

This PR is intended to solve problems caused by System.Text.Json version 7.x not actually being compatible with Net 6.0 (nor [presumably] older).

I am assuming System.Text.Json 6.x works with netstandard2.0 since I see no issue to the contrary and all tests are passing.
If not, this shouldn't make the solution any worse for them and the solution should be as good as it was for them before this NuGet was revised to use System.Text.Json version 7.x.

(If System.Text.Json 6.x does not work for netstandard2.0, this is beyond the scope of this PR as well as beyond the scope of what I am presently willing or able to fix. If this is still needed, it may even mean using Newtonsoft.Json as an alternative, which would could add a lot of complication to the codebase.)

[ngocnicholas]

Changes cannot be used because with them in the project files, the build will fail in my VS 2022 (17.2.4), .NET Framework 4.8 build environment. Please test your improvements and then reopen this pull request again. Thank you for your contribution.

[nilvon9wo]

@ngocnicholas ,
I don't have or want to have every .NET environment on my system, nor am I looking to devote much of my time to your project.

This PR is a demonstration of how you can and should fix your project which does not work properly with .NET 6.0 because you've updated it to reference a .NET 7.0 project.

Regardless of your tests, I would be incredibly surprised if it works with .NET 4.8 or any version of .NET older than 7.0.

If you care to fix this, there should be sufficient information in this branch for you to add support for whatever additional targets you feel are necessary or desirable to support and the work should be trivial.