[TASK] Rename cache t3lib_l10n to l10n

As t3lib doesn't exist anymore, get rid of this naming schema.

Change-Id: I525db9a511813e1e517c1363e29894b34b80ad48
Resolves: #55149
Releases: 6.2
Reviewed-on: https://review.typo3.org/26934
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Reviewed-by: Marcin S?gol
Tested-by: Marcin S?gol
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer