Make STJ Conditional based on TFM

[thompson-tomo]

Is there an existing issue for this?

• I have searched the existing issues

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 not have an explicit dependency on System.Text.Json for net 8.

Describe alternatives you've considered
Accept the additional dependency

Additional context
n/a

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[thompson-tomo]

Still valid.