[TASK] Create a Getter for CacheManager inside Datahandler

At the moment the CacheManager is created by
GeneralUtility::makeInstance(
  'TYPO3\\CMS\\Core\\Cache\\CacheManager'
); within the code. This makes the particular method untestable.

Solution: Create a Getter which creates and returns the instance.
This getter can be mocked.

Resolves: #60059
Releases: 6.2, 6.3
Change-Id: I9f83ec017da94778e542410cc9d03faacddc245c
Reviewed-on: https://review.typo3.org/31288
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik