Drop UBUNTU_CODENAME from /etc/os-release

[Benjamin Drung]

Hi everyone,

Bug #1520154 introduced UBUNTU_CODENAME, but the final name that landed
in the os-release specification is VERSION_CODENAME. Ubuntu specifies
VERSION_CODENAME since Ubuntu 16.04 "xenial" (see bug #1598212).

So let's finally drop UBUNTU_CODENAME from os-release starting with
Ubuntu 24.10 "oracular": https://launchpad.net/bugs/2065040

-- 
Benjamin Drung
Debian & Ubuntu Developer