[TASK] Avoid getLL() usages in remaining system extensions

Using sL() instead of getLL() shows direct dependencies
on labels and their label files within the code.

This change replaces all usages in
* backend
* linkValidator
* rte_ckeditor

with sL() calls.

Resolves: #100675
Relates: #100668
Releases: main
Change-Id: I76b030263987951f232caf50e7d2cd6526480215
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78759


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