[TASK] Bring back module menu accordion arrows

To visualize that the module groups can be expanded and collapsed, the UX
team decided that the arrows are required. This brings them back on the
right with a very light gray to avoid distractions.

Resolves: #53651
Releases: 6.2
Change-Id: Ie289b59dfb24077ed99964e7e944c8dfd443af59
Reviewed-on: https://review.typo3.org/25415
Reviewed-by: Jost Baron
Tested-by: Jost Baron
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer