PR Summary

This implicitly ups the minimum requirement for PS Core to 6.2.0 (because PowerShell cannot load a newer version of Newtonsoft than the version that it ships with). This is OK because PS 6.1 will go out of support in a week anyway.
I had to change the AppVeyor image to a newer one to get PS 6.2.0. If that AppVeyor had a newer patch of 6.2 then we could've even upgraded to Newtonsoft 12.0.2 (PowerShell 6.2.1 started shipping with this patch)

PR Checklist

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