[BUGFIX] Fix incorrect variable usage in BackendUserLanguageMigration

This was most likely a very old copy-paste error.

Used command:

> ./Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #98025
Releases: main, 11.5
Change-Id: Idacf7ca70895a9fd1be9c765e714a68ab6e44032
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75292


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>