Use polymorphic JSON serialization for WebSocket events

`System.Text.Json` has introduced polymorphic JSON serialization which allows to serialize and deserialize objects without writing custom converter. This can make the user faced API simpler and easier to write new code. To get this working .NET 7.0 or higher is required.

Implementing this can break with old versions of the web server as this changes the handling of events completely.

More information: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-7-0

Start by locating the WebSocket event serialization and deserialization path, then check the project's target framework and current event handling. Read the linked System.Text.Json polymorphism guidance before assessing the .NET 7 requirement. Done means polymorphic serialization handles WebSocket events and the impact on older web-server versions is addressed.

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