Replace Manatee Json in old editor

[RonnyB71]

Description

The old editor code will continue to live for as long as we need to support Seres or XSD centric data modelling capabilities. We need to consider if we should replace Manatee.Json as-is with JsonSchema.Net for this flow as well since Mantee.Json is abandoned an not actively maintained.

Considerations

This depends on how long we need to support the old model editor and how much it "hurts" to have two libraries for managing Json Schemas.

This is not the same as #6640 as this is basically a cleanup task after the previous

Acceptance criteria

• [ ] No references to Manate.Json
• [ ] JsonSchema.Net is used to handle Json Schemas

Development tasks

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Test

Add test cases here as checkboxes that are being tested as part of the changes.

Definition of done

• [ ] Documentation is updated (if relevant)
  • [ ] Technical documentation (docs.altinn.studio)
  • [ ] User documentation (altinn.github.io/docs)
• [ ] QA
• [ ] Manual test is complete (if relevant)
• [ ] Automated test is implemented (if relevant)
• [ ] All tasks in this userstory are closed (i.e. remaining tasks are moved to other user stories or marked obsolete)

[RonnyB71]

If we decide to drop the old editor then this is not needed.

[nkylstad]

Closing this, reopen if we see the need.