[BUGFIX] Adjust category exception message to namespaces

The exception message in ExtensionManagementUtility::makeCategorizable
now refers to namespaces instead of t3lib.

Fixes: #42996
Releases: 6.1, 6.0

Change-Id: Ifd52a2fb96335fa9bde6ec98b6ddcf8dd4c6bf74
Reviewed-on: https://review.typo3.org/16451
Reviewed-by: Steffen Ritter
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack