[BUGFIX] Extbase must not use legacy flash message handling

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

Also introduces a compat layer.

Releases: 6.1
Resolves: #47196
Change-Id: I4dbee79e9181c5ecbece4549039114f58ea6f33a
Reviewed-on: https://review.typo3.org/19944
Reviewed-by: Alexander Schnitzler
Tested-by: Alexander Schnitzler