Add additional TFM to eliminate explicit dependency

[thompson-tomo]

Is your feature request related to a problem? Please describe.
I want to minimise dependencies in my project by utilising framework dependencies wherever possible

Describe the solution you'd like
I want the package to have an additional TFM (net 6) added so that the explicit dependency on System.Text.Json can be removed.

Describe alternatives you've considered
Accept the additional dependency

Additional context
n/a