Replace Newtonsoft.Json with System.Text.Json

### Description

To avoid unnecessary third party (other than Microsoft) we should move away from Newtonsoft.json and use System.Text.Json from Microsoft instead.

### Additional Information

_No response_

### Tasks

_No response_

### Acceptance Criterias

- [ ] Models are serialized the same way with regards to casing ie. camelCase
- [ ] Any references to Newtonsoft is removed
- [ ] The serialization should relax on the casing when receiving data, ie. be case insensitive. But be strict when sending data, ie. enforce camelCase.

Start by locating every Newtonsoft.Json reference and the model serialization and deserialization entry points in the repository. Compare the resulting behavior for camelCase output and case-insensitive input, then verify that no Newtonsoft references remain.

Written by the indexing model from the issue text.