[TASK] Add (>) icon to execute button (Scheduler)

Scheduler backend module comes with an execute button.
Add icon to the button.

Change-Id: I842683c4b332a87653474237a893dd65d5bd1b5d
Resolves: #55789
Releases: 6.2
Reviewed-on: https://review.typo3.org/27451
Reviewed-by: Marcin Sągol
Tested-by: Marcin Sągol
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters