[CLEANUP] Cleanup ConfigurationManager.php

This mainly fixes CGL violations:

* missing @return annotations
* variables must be lower camel case
* line lengths
* methods should only have 1 return statement
* import classes
* comments must be on seperate line
* remove superfluous empty lines
* remove superfluous comments
* intentions of blocks
* remove unused local variables
* use single quotes instead of double quotes

Resolves: #58246
Releases: 6.2
Change-Id: I57074a3d435039286d4f8000059266445a7a71ae
Reviewed-on: https://review.typo3.org/29682
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen