[TASK] Flatten row-headers

Flattens the 3d gradient row-headers as discussed in UX team.
Following the new page module column style.

Change-Id: I4dea32d0e4ed021baa402559eeda91f9293b5abf
Fixes: #43282
Releases: 6.1
Reviewed-on: https://review.typo3.org/18715
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn