[BUGFIX] Fix type annotations concerning storage folder identifiers

Storage folder identifiers can also be int-like strings, which causes
them to automatically get cast to int when used as array keys.

Our types annotations need to reflect this in order to keep developers
and static analysis for taking those array keys to be always strings
(which can lead to type errors and has recently done so).

Resolves: #97858
Relates: #97851
Relates: #97711
Releases: main, 11.5
Change-Id: If0fbfab5792446cfe10564803d8d540dd3cc6bb5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75033


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>