[FEATURE] Update FlashMessage API calls in DefaultController

Replace deprecated:
$this->flashMessageContainer->add()
with:
$this->addFlashMessage()

Resolves: #55068
Releases: 6.2
Change-Id: Ib2101ca7e00fe1e971b9daea671dcecb9aba4512
Reviewed-on: https://review.typo3.org/26880
Reviewed-by: Francois Suter
Tested-by: Francois Suter