Remove usage of FormatterServices.GetUninitializedObject as it is obsolete in .NET9

[mo-russo]

https://learn.microsoft.com/en-us/dotnet/fundamentals/syslib-diagnostics/syslib0050#workaround

From the page above, we should update the JsonObjectContract.GetUninitializedObject() function to use the following code:

internal object GetUninitializedObject()
{
    // we should never get here if the environment is not fully trusted, check just in case
    if (!JsonTypeReflector.FullyTrusted)
    {
        throw new JsonException("Insufficient permissions. Creating an uninitialized '{0}' type requires full trust.".FormatWith(CultureInfo.InvariantCulture, NonNullableUnderlyingType));
    }

#if NET5_0_OR_GREATER
    return System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(NonNullableUnderlyingType);
#else
    return System.Runtime.Serialization.FormatterServices.GetUninitializedObject(NonNullableUnderlyingType);
#endif
}