Migrate from `Newtonsoft.Json` to `System.Text.Json`

For a long time `Newtonsoft.Json` was the best JSON library for .NET. However, since `System.Text.Json` was added in .NET Core 3.0, it's clearly the future. It's a core part of .NET, so there's no need for an additional dependency, and it's far more performant than `Newtonsoft.Json`

A couple of things to be aware of:
- Should we explicitly add [`JsonPropertName` attributes][3] to object properties?
- Should we standardize on [Camel Case][4] (`exampleVariableName`) over [PascalCase][5] (`ExampleVariableName`)?
- Currently we use a mix of both
- `Newtonsoft.Json` can accept both by default, can `System.Text.Json` too?
- Migrate to asynchronous methods instead of synchronous?
- [`SerializeAsync`][6]
- [`DeserializeAsync`][7]

References:
- [`Newtonsoft.Json` to `System.Text.Json` migration guide][1]
- [What's next for `System.Text.Json`][2]

[1]: https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to?pivots=dotnet-6-0
[2]: https://devblogs.microsoft.com/dotnet/whats-next-for-system-text-json/
[3]: https://docs.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonpropertynameattribute?view=net-6.0
[4]: https://en.wikipedia.org/wiki/Camel_case
[5]: https://en.wiktionary.org/wiki/Pascal_case
[6]: https://docs.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializer.serializeasync?view=netcore-3.1
[7]: https://docs.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializer.deserializeasync?view=netcore-3.1

The issue names no files, tests, or entry points; start by inventorying the repository's Newtonsoft.Json usage and reading the linked System.Text.Json migration guide. Resolve the open decisions about property naming, attributes, compatibility, and asynchronous APIs, then verify that all serialization behavior and dependency usage have been migrated.

Written by the indexing model from the issue text.