[BUGFIX] Simplify Apcu cache backend identifier calculation

APCu cache backend is a shared memory per user and
typically bound to the executing posix user-id.
Calculating different cache identifiers per user
is useless since those won't share the same shm.

Resolves: #96232
Releases: master, 11.5, 10.4
Change-Id: If6b6a744828d06d55f2c9b5656aef48193da7384
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72511


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>