[TASK] Centralize logic from PageLayoutController->getExistingPageTranslations() in BackendUtility

The logic of PageLayoutController->getExistingPageTranslations() was
moved to BackendUtility::getExistingPageTranslations() to make the
code reusable in other places.

Resolves: #103274
Releases: main
Change-Id: I2fbc596ff33356e1dd4c39c11a7ccea409af945e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83201


Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Frank Nägler <frank.naegler@typo3.com>