[TASK] Update t3lib mentions

Some core parts still mention t3lib classes after namespaces,
mostly in comments.

Related: #47138
Releases: 6.1

Change-Id: Ic3cb7119503e69b0914ef51b80360266ff3c2df8
Reviewed-on: https://review.typo3.org/19808
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring