[TASK] Use Sortable.js in FlexForm

This patch replaces jquery-ui/sortable with Sortable.js which has been
introduced with the refactoring of the IRRE component.

Additionally, some styling is added to improve the visual indication of a
current sorting in progress.

Resolves: #90561
Releases: master
Change-Id: If2c15b16aa21ff9abbc91bdb00544a16b383a591
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63477


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Georg Ringer <georg.ringer@gmail.com>