PR Summary

Before the change we used Newtonsoft Json.NET JObject.Parse() method to validate Json. The method is not best choice to validate Json because it doesn't parse valid primitives.

As result we:

• migrate the cmdlet to new Core API - System.Text.Json. JsonDocument.Parse()
• change old tests to follow JSON standard (use double quotes as delimiters instead of single quotes)
• add new tests
• enable nullable reference types to follow best practice

Formally it is a breaking change because Newtonsoft Json.NET JObject.Parse() accepted single quote delimiters.

PR Context

Fix #11384

Need review #5797

.Net docs https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to

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