[TASK] Cleanup extbase persistence backend class

This change cleans up extbase persistence backend class, by:

* using constructor property promotion
* using strict types and return types where possible
* removing self explaining method doc comments

Resolves: #104006
Releases: main
Change-Id: Idb9d6fe252095d3ded49e752becc09957d8ee63e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84532


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>