[BUGFIX] Determine default severity for modals correctly

This patch fixes the determination of the default severity for modals.
This ensures that the modal's header and its default button receive
the same coloring.

Resolves: #80677
Related: #80579
Releases: master
Change-Id: I7bfc5bff21a28318b2fea03f5fe534264e47cbb0
Reviewed-on: https://review.typo3.org/52356


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>