[TASK] Lower memory use in ArrayUtil->mergeRecursiveWithOverrule

Iterate on the original array instead of an array_keys()-copy in
ArrayUtil->mergeRecursiveWithOverrule which lowers memory usage and
enhances performance.

Resolves: #63812
Releases: master, 6.2
Change-Id: I71acc9e1e730f1e782d0e74acd45637ce1b9f08e
Reviewed-on: http://review.typo3.org/35535
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>