[TASK] Update t3lib mentions

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

Related: #47138
Releases: 6.1

Change-Id: I6e732159f92fb539462eba93f8578d90be097327
Reviewed-on: https://review.typo3.org/19805
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring