[TASK] Deprecate Environment::getBackendPath()

As the hard-coded "/typo3/" path might change later,
it is good to rely on the BackendEntryPointResolver
for future development.

Resolves: #99638
Releases: main
Change-Id: I9cf0eb52bcb22a4656eddccf72eb8b1456a458d7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75859


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>