I doubt that this is a PHP 8.3 issue. That's a standard PHP global var…
Do you see a placeholder maintenance page when accessing your site? If that is intended behaviour after the first setup run. Please see the notes here: https://www.zenphoto.org/news/installation-and-upgrading/
Thet site is proably on maintenance mode during and after setup runs. You have to disable it yourself or change the defaults. There is a visible link about that on the backend and on the admin toolbar.
If you indeed see the code displayed something is wrong on your server. A link would probably help to help.
It's other issues but perhaps somehow related, please see: https://forum.zenphoto.org/discussion/1411336/cant-setup-call-to-a-member-function-lock#latest