[CLEANUP] Rename AbstractContentObject::getContentObject()

The method is renamed to getContentObjectRenderer(), as this better
reflects what it actually does. The old method is marked as deprecated
and scheduled for removal in v9.

Change-Id: Idaaba8856f7cca31bdc1d5c301a221dee2059048
Resolves: #68748
Releases: master
Reviewed-on: https://review.typo3.org/42272


Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>