Add withdrawal rationale for "Extending Long Tasks API to Web Workers" explainer

Update the Withdrawn section in README.md to include the reason for withdrawing
the "Extending Long Tasks API to Web Workers" explainer. The note clarifies that
the team is proposing the Delayed Messages API as an alternative approach.

Author: Joone Hur

README.md

@@ -254,5 +254,6 @@ We'd love to hear your feedback!
 | [Version History](VersionHistory/explainer.md) | | 2023-07-17 |
 | [Confirmation of Action](Accessibility/ConfirmationOfAction/explainer.md) | This effort is being pursued in a similar proposal, [AriaNotify](Accessibility/AriaNotify/explainer.md)| 2023-11-13 |
 | [PerformanceNavigationTiming Protocol Launch](PerformanceNavigationTimingProtocolLaunchExtensions/explainer.md) | | 2024-04-22 |
+| [Extending Long Tasks API to Web Workers](LongTasks/explainer.md) | Rather than extending the Long Tasks API to monitor long tasks in web workers, we propose introducing [the Delayed Messages API](DelayedMessages/explainer.md) | 2025-06-09 |