[BUGFIX] Fix handling of deprecated cache configuration

When initializing the configuration for a cache any existing
configuration under its old name (cache_ prefixed) is applied
as an additive override now. This ensures that basic configuration
like groups are preserved and not removed with the formerly correct
way to adjust cache-config.

Resolves: #91306
Releases: master
Change-Id: Ic862f80263f410688d2dffb7c13948c1c40488a3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64407


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>