[Bug] Cancellation token is REQUIRED when starting .NET worker

[donald-pinckney]

See worker setup code in references/dotnet/dotnet.md. The call to ExecuteAsync requires an argument for stoppingToken (https://dotnet.temporal.io/api/Temporalio.Worker.TemporalWorker.html#Temporalio_Worker_TemporalWorker_ExecuteAsync_System_Threading_CancellationToken)