replace newtonsoft with system.text in MultiUrlPickerValueEditor

[CyberReiter]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Replaced Newtonsoft with System.Text.Json to make the MultiUrlPicker more performant.

How to test

• Create a content type with a multiurlpicker, make sure that the configuration gets saved correctly
• Create a page based on this content type and try to access properties of the Multiurlpicker

[github-actions]

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[nul800sebastiaan]

Hello again @CyberReiter - I thought I'd closed all of the Newtonsoft replacement PRs already, turns out this one was still open, sorry!

As before: I will close this one and the related Newtonsoft updates. There are subtle differences between Newtonsoft and System.Text.Json that will lead to unexpected output and breaking things.

We are planning to remover Newtonsoft completely from the solution in Umbraco 13: umbraco/Announcements#9

Thanks for the PR!