[TASK] Adapt skinning extension stylesheet paths

In order to clean up EXT: t3skin the standard paths should
be adapted to follow extbase/fluid conventions.

Change-Id: Ic43a42e13c7656064743c936b678a5172e45209b
Resolves: #51134
Releases: 6.2
Reviewed-on: https://review.typo3.org/23144
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters