Add a Unit Test for MamJsonParser and upgrade it to use System.Text.Json (#9797)

Context: f6f11a5
Context: #9229

Commit f6f11a5 added support for type and member remapping, to
assist the InTune team.

The `MamJsonParser` type added in f6f11a5 uses Newtonsoft.Json, and
is one of very few uses of Newtonsoft.Json.

Update `MamJsonParser` to use System.Text.Json, so that we can
eventually remove the Newtonsoft.Json dependency from our installs.

Additionally, add a unit test (shock!) for the `<MamJsonToXml/>` task.

The input `mam.json` test JSON data comes from the
`content/MonoAndroid10/remapping-config.json` file within the
[`microsoft.intune.mam.remapper.tasks` NuGet package][0], while
`mam.xml` is the expected output from `tools/remap-mam-json-to-xml`.

Co-authored-by: Jonathan Peppers <[email protected]>

[0]: https://www.nuget.org/packages/Microsoft.Intune.MAM.Remapper.Tasks/0.1.4635.1

Author: dellis1972