[BUGFIX] Cast replacement values to strings

Avoid exceptions due to wrong type in the link validator module.

Resolves: #101352
Releases: main, 12.4, 11.5
Change-Id: Ic54b38e2fad6c203526fe27fa1b2ea6815f0578f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80028


Reviewed-by: Stefan B�rk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan B�rk <stefan@buerk.tech>