Update javadoc to take virtual threads into account

[wilkinsona]

Affects: 6.1.0.M2

Similar to the logging changes tracked by #30902, there are some places where the javadoc should also be updated to take virtual threads into account, including:

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/AsyncSupportConfigurer.java

Lines 58 to 60 in 0d8a843

	* <p>By default only a {@link SimpleAsyncTaskExecutor} is used. However when
	* using the above two use cases, it's recommended to configure an executor
	* backed by a thread pool such as {@link ThreadPoolTaskExecutor}.

and perhaps

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java

Lines 402 to 404 in 0d8a843

	* <p>By default a {@link SimpleAsyncTaskExecutor} instance is used.
	* It's recommended to change that default in production as the simple executor
	* does not re-use threads.

These are just two places that I noticed and I haven't done an exhaustive search so there may be others.