[TASK] Streamline usage of `Persistent.loadFromServer()`

Previously, the internal method `loadFromServer()` from the module
`@typo3/backend/storage/persistent` fetched the data, wrote it into an
internal property `data` and also returned it. The code is now
simplified to only return the response.
Setting `data` is now done within the public `get()` method, which is
now also simplified a bit.

Resolves: #103886
Releases: main
Change-Id: I975a6ad0c2761afdadadfdc05dfa9c11a47779fa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84374


Tested-by: Andreas Nedbal <andy@pixelde.su>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Marvin Buchmann <marvin.buchmann@typo3.com>
Reviewed-by: Willi Wehmeier <wwwehmeier@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Andreas Nedbal <andy@pixelde.su>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>