Make Workhorse gracefulShutdownDelay default to shutdown.blackoutSeconds

What does this MR do?

With !4586 (merged), we are disabling the preStop hook for Workhorse when the health check listener is enabled so that Workhorse can signal immediately that the pod is no longer ready.

However, we still need to observe the blackout time to ensure ongoing requests complete. Previously this would require the admin to set the gracefulShutdownDelay properly, but we should just observe the blackout time if it's available.

Related issues

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