[TASK] createEncryptionKey always returns key with 96 characters

Update the createEncryptionKey method to use the same underlying
call as its copy in the eID script.

Change-Id: I666a3620080fd0f5281e899250f5c710e2550173
Resolves: #29220
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/21750
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe