[TASK] Clean up BackendConfigurationManager

A first code clean up of this class streamlines
the properties and removes some rather obvious
stuff. The unit tests receive an overhaul.

Resolves: #98581
Related: #98578
Releases: main
Change-Id: I9a6db5015c976986fde24f721e5c2fd8cc1d3f04
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76090


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>