Improve JSON serialization and deserialization

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• Use JsonContent.Create() for requests in order to avoid the intermediate string serialization.
• Use ReadFromJsonAsync() for responses in order to avoid the intermediate string serialization.

Also update target framework from .NET 6 (unsupported) to .NET 8 (current LTS).

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Hey Ilya,
Just a quick reminder to take a look at that pull request when you get a chance. No rush, I just wanted to make sure it didn’t slip through the cracks. Let me know if you have any questions or need anything from me.

Thanks!