Switch more over to System.Text.Json

[dellis1972]

• JsonElement is read only. This is why we convert them to JsonNode.
• JsonNode.Parse does NOT have the same features as JsonDocument.Parse. e.g you CANNOT tell it to ignore trailing comma's. It will just throw an exception. This is the reason why we have to load the .json via JsonDocument and then call the extension method .ToNode () on the Root JsonElement to convert it to a JsonNode. (We use JsonNode to do the merging).

[dellis1972]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jonathanpeppers]

Wait, is System.Text.Json in the pack?

It would be needed for .NET framework.

[dellis1972]

No idea. Probably. We need to figure out how to get #9727 and #8746 working so we can test under .NET framework to catch all this stuff.

[jonpryor]

WIP commit message:

[Xamarin.Android.Build.Tasks] Remove use of Newtonsoft.Json (#9819)

Context: https://github.com/dotnet/android/issues/9229

Remove use of Newtonsoft.Json from `Xamarin.Android.Build.Tasks.dll`.

Notes:

  * `JsonElement` is read only.  This is why we convert them to
    `JsonNode`.

  * `JsonNode.Parse()` does NOT have the same features as
    `JsonDocument.Parse()`, e.g you CANNOT tell it to ignore trailing
    comma's; it will just throw an exception.  This is the reason why
    we have to load the `.json` via `JsonDocument` and then call the
    extension method `.ToNode ()` on the Root `JsonElement` to convert
    it to a `JsonNode`.  (We use `JsonNode` to do the merging).

Question: does this PR fix #9229?