The PHP-version on the Univention server does not matter.
More important is the PHP-version in the Nextcloud docker container.
I am using Univention Server 5.0-5 with PHP 7.3.31 and the Univention Nextcloud 25.0.9 in a Docker container. Insinde the docker container there is PHP 8.1.2 running.
So the Nextcloud developers have the recent PHP version available.