Use AbstractAotProcessor.AOT_PROCESSING constant instead of "spring.aot.processing"

[philwebb]

spring-projects/spring-framework#33388 is now fixed so we can replace our current duplicate strings.

[ngocnhan-tran1996]

Can I work on this?

[wilkinsona]

Thanks for the offer, @ngocnhan-tran1996, but this can't be done until we've moved onto Spring Framework snapshots. It's probably also easier for someone on the team to take care of it as it's just a simple search and replace and the overhead of processing a PR for that is quite high.