Newtonsoft.Json -> System.Text.Json

[mpreyskurantov]

Changelog:

Structural changes:

• Remove the Newtonsoft.Json ref from a library, move it to explicit consumers projects ("Tests.Common" + all its satellites and "Tests.NET4" separately) and "Sample" instead, bump to the latest version;
• Add a build configuration symbol / constant to highlight / isolate Newtonsoft.Json tests (for further moving to a separate project);
• Add the System.Text.Json cross-target ref, bump the net4 target to a minimal supported version in affected projects accordingly;
• Clean up usings / namespaces in affected files.

Behavior changes:

• Implement migration at the serialization / de-serialization level only to minify impact to cross-platform / DBMS-agnostic Data Layer / API. It is primarily related to DataSourceLoadOptionsParser class, which is usually used of the ASP.NET MVC / Core Model Binder.
• There is no straightforward way to implement a good converter for the "Filter" IList (i.e., a single part of API without the explicit type param), because System.Text.Json types rely on a set of private var / flags and non-obvious "goto" operators internally:

https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs
https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.cs

That is why, it is kept the default IList traversing with further unwrap its operands tree, whose items (primitives) wrapped with JsonElement.

• Ignore (for now) some non-supported scenarios (covered with tests) until their proving or may be agreed bump to the net7 target, where advanced functionality for this purpose is available.

Testing:

Tested against the "Sample" project in this solution (JS dxDataGrid + ASP.NET MVC Backend / Controller) and a modern Blazor DxGrid https://demos.devexpress.com/blazor/Grid/DataBinding/LargeQueryableAsHttp + Data Editors (based on a legacy DxDataGrid).