So 12.1 is out but have you upgraded any of your packages yet? The /etc/debian_version file comes from the base-files package. On my up to date system, it’s showing 12.1 in the file and the package version is 12.4+deb12u1 as I can see from dpkg -l base-files.
Make sure to do an apt update and then do an apt upgrade -s to do a dry-run to see what packages would be upgraded. I’m guessing the base-files package hasn’t been updated.