[TASK] Ensure unique values in filemounts permissions

Ensure unique values in BackendUserAuthentication::groupData['filemounts']
by processing it through the StringUtility::uniqueList() function.

Prior to this change, values were not always unique due to the
combination of permissions for backend users and groups.

Resolves: #102857
Releases: main, 12.4
Change-Id: I37b4021e22cd3dfaa5323fe6713fae321b72173d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82481


Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>