If cron is failing, but Nextcloud is otherwise operating, than it’ s probably not because of your PHP version.
If you’re already upgraded to NC26 than you’re already using at least PHP 8.0 (v8.1 I believe if you’re using ncp-update-nc).
If you’re running NCP, you should be able to use NCP’s built in update process and not need to mess with upgrading PHP at all on your own unless something gets really messed up.