[TASK] Deprecate GeneralUtility::array_merge()

1. Mark and log as deprecated
2. Change function call throughout Core to native php '+' operator

Resolves: #64596
Releases: master
Change-Id: I389a794352d2e5f37f284448884cd6077fb018c8
Reviewed-on: http://review.typo3.org/36377


Reviewed-by: Zbigniew Jacko <z.jacko@macopedia.pl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>