[TASK] Deprecate StringFrontend cache frontend

Can be drop-in replaced by VariableFrontend. There are no
occurrences in core code except for in tests and comments.
Configurations using this frontend can be automatically
migrated on-the-fly. Such auto-migration is not part of this
patch - this patch only touches the PHPDOC and documents
the deprecation.

Change-Id: I3b4b4979534857fa30b56d9967e8f919fc046ab9
Resolves: #81434
References: #81432
Releases: master
Reviewed-on: https://review.typo3.org/53084


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>