[TASK] Add todo for strictly typing class UnableToLinkException

The constructor of UnableToLinkException should be strictly
typed, since the method getLinkText() is strictly typed.
Also make the variable declaration of $linkText strictly typed.
It will throw an error if $linkText isn't a string.
Added a todo for TYPO3 v13.

Resolves: #100119
Related: #100106
Releases: main
Change-Id: Ifbb9a3f5b4aa1fd1588eb0596731377f41bc5ffd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78072


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>