[BUGFIX] Call to FlashMessageQueue::addMessage() method in extbase

Extbase still uses the addMessage method, which is deprecated.
The official API is now enqueue(), let's use this one.

Resolves: #52488
Releases: 6.1, 6.2
Change-Id: I85097ee53ded9c2d2ccdc6c240667ddaa3768999
Reviewed-on: https://review.typo3.org/24280
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind