[BUGFIX] Rename hook in VariableFrontend.php

Due to the namespace changes the hook in
typo3\sysext\core\Classes\Cache\Frontend\VariableFrontend.php was
renamed. This patch changes name back to the one before.

Resolves: #53707
Releases: 6.2, 6.1, 6.0
Change-Id: Ibb86188b38aba9ca7e7e2670f843234fae16fa87
Reviewed-on: https://review.typo3.org/25466
Reviewed-by: Alexander Opitz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters