fix: Wrong data is displayed at RestApi page's examples

[filzrev]

This PR intended to fix problems on RestApi page's examples. (and related test snapshots)
(e.g. https://dotnet.github.io/docfx/seed/restapi/contacts.html)

This problem is caused by following lines.

docfx/src/Docfx.Build.RestApi/SwaggerModelConverter.cs

Line 91 in 3342549

Content = example.Value != null ? JsonUtility.Serialize(example.Value) : null,

Currently JsonUtility.Serialize<object>(JObject model) use System.Text.Json code path.
It need to use NewtonsoftJson serializer instead.