Update Newtonsoft json to 12.0.1 due to PowerShell 6.1 going out of support

[bergmeister]

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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• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
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