Add support for PHP 8.5

[stweil]

Description

Problem

PHP 8.5 is the current PHP release and should be supported.

Proposed Solution

Update code to support it.

Target Version

Current main branch.

[stweil]

This needs an update for ubl/php-iiif-prezi-reader, see ubleipzig/php-iiif-prezi-reader#6 and a new release with this update. The update and the release could also be added in https://github.com/kitodo/php-iiif-prezi-reader/.