I ran into this before, the PPA (ppa:ondrej/php) doesn't provide PHP 8.3 for Ubuntu 20.04. The highest available is usually 8.2 on focal. If you specifically need 8.3, you’ve got a couple of options:
Upgrade to Ubuntu 22.04 or newer: the PPA provides 8.3 for jammy (22.04) and later.
Build PHP 8.3 from source: a bit more work, but possible if upgrading isn't an option.
Use Docker: you can spin up a container with PHP 8.3 easily.
If you try to build from source, make sure you grab all the necessary dependencies first, otherwise it can get messy.