Affect: Spring Boot 3.2.2, spring-messaging-6.1.3

Make it possible to set SimpleAsyncTaskExecutor for a channel through ChannelRegistration.taskExecutor(taskExecutor), not only ThreadPoolTaskExecutor, to support Java 21 virtual threads. And it might be worth making the default use of SimpleAsyncTaskExecutor as channel's TaskExecutor when spring.threads.virtual.enabled=true in Spring Boot 3.2