[BUGFIX] Fluid must not use legacy flash message handling

Since the core introduced a new flash message handling the
handling in Fluid has to be adjusted as well.

Releases: 6.1
Resolves: #47214
Change-Id: I490f77687befa1a62db0718f828fce1e68ebdfff
Reviewed-on: https://review.typo3.org/19972
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Alexander Schnitzler
Tested-by: Alexander Schnitzler