[TASK] Optimize the spacing in scheduler list

With #100663 the overall spacings were adjusted. With this patch we
align the spacing between scheduler group panels with the spacing
between record list items.
Also, a margin above the 'Unused task groups' section has been
reduced.

Resolves: #100735
Related: #100663
Change-Id: I618350f1bfa79cf67f781766a2a647c05b619cfb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78813


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>