[BUGFIX] Avoid deprecated PHP function `utf8_decode`

[oliverklee]

utf8_decode has been deprecated, and calling it in PHP >= 8.2 throws a deprecation warning.

This change bringt the two affected calls in line with what we already have in main in this commit: 134418314d6db55d463bd0aa882942ba10710c1b

https://www.php.net/manual/en/function.utf8-decode.php