Fixes: https://github.com/spring-projects/spring-integration/issues/9706
Previously, if a `MessageGroupProcessor` returns a collection of payloads,
the `AbstractCorrelatingMessageHandler` has failed with the `IllegalArgumentException`
stating that only collection of messages is possible.
From now on such a restriction is eliminated and returned collection of payloads
is emitted as a single reply message from the aggregator.
* Add `AbstractCorrelatingMessageHandler.isResultCollectionOfMessages()`
to return `true` only if result is a collection of messages, treating them as a "partial sequence".
* Deprecate `AbstractCorrelatingMessageHandler.verifyResultCollectionConsistsOfMessages()`
since it is out of use now.
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