[TASK] Migrate getPagesTSconfig to runtime cache

BackendUtility::getPagesTSconfig() uses local
method static properties as cache.
This prevents dedicated flushing, which may
be needed in rare cases to prevent for instance
from exceeded memory during huge imports.
The patch switches that to cache framework
cache_runtime which is already used at other
places within BackendUtility.

Resolves: #82473
Related: #63629
Releases: master, 8.7
Change-Id: I4d54819f05a6c7b8ce175bc827b626277e2d4c04
Reviewed-on: https://review.typo3.org/54133


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>