[TASK] Deprecate IconUtility::getIconImage

IconUtility::getIconImage
is not used anymore in the core, so deprecated this function.

Change-Id: Ic9d33160d0ca802dcdec929400f7919e39c8aec8
Resolves: #22706
Releases: 6.1
Reviewed-on: https://review.typo3.org/12903
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack