[DOCS] Adjust scheduler API docs to reflect new SchedulerTaskRepository

With https://review.typo3.org/c/Packages/TYPO3.CMS/+/78197 the
methods for reading and writing to the database were moved to
a dedicated repository. The documentation does not reflect this
change in the "Scheduler API" chapter.

Resolves: #101062
Resolves: https://github.com/TYPO3-Documentation/Changelog-To-Doc/issues/526
Resolves: https://github.com/TYPO3-Documentation/Changelog-To-Doc/issues/405
Related: #101040
Related: #100247
Releases: main, 12.4
Change-Id: I871bd54e41882c35eaaeeb325c8bed5c2dd1b2e7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79378


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