[!!!][TASK] Remove deprecated Environment method

Removed public methods:
* Environment::getBackendPath

Resolves: #100997
Relates: #100963
Releates: #99638
Releases: main
Change-Id: I1a23c314f660b74836c0858e190c48090f195e62
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79260


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>