Support spring 6.1.x lifecycle changes/late-shutdown for pubsub publisher scheduler

[jayakumarc]

There has been a significant revision of ThreadPoolTaskScheduler/Executor lifecycle capabilities as part of the spring-6.1.x release. It includes a concurrently managed stop phase for ThreadPoolTaskScheduler/Executor, favouring early soft shutdown. As a result, the ThreadPoolTaskScheduler, which is used for pubsub publishing, now shuts down immediately on ContextClosedEvent, thereby rejecting any further task submissions (#2721).

This PR aims to retain the default behavior of spring-6.1.x but provides config options to leverage the lateShutdown of the underlying ThreadPoolTaskScheduler, such as:
spring.cloud.gcp.pubsub.publisher.executor-accept-tasks-after-context-close=true.

References:

1. Graceful shutdown does not cancel @Scheduled tasks spring-projects/spring-framework#32109 (comment)
2. spring-projects/spring-framework@b12115b
3. spring-projects/spring-framework@a2000db
4. ScheduledAnnotationBeanPostProcessor: graceful shutdown should not interrupt currently running jobs spring-projects/spring-framework#31019 (comment)
5. https://github.com/spring-projects/spring-framework/blob/996e66abdbaad866f0eab40bcf5628cdea92e046/spring-context/src/main/java/org/springframework/scheduling/concurrent/ExecutorConfigurationSupport.java#L482

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[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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