[SECURITY] Prevent XSS in scheduler form

The class name is submitted in a hidden form and is susceptible to XSS.
The patch introduced htmlspecialchars to prevent XSS possibility.

Resolves: #57603
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I4979e66f28a581e168c56d91327a1bbe2672448d
Reviewed-on: https://review.typo3.org/29154
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes