[TASK] Update t3lib mentions

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

Resolves: #47138
Releases: 6.1

Change-Id: I0775fc10f8c901a02533e5bc0fe1c6aa2276758b
Reviewed-on: https://review.typo3.org/19803
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn