Updating the LXC to Debian 12 manually could potentially cause a loss of functionality.
Here’s an approach to upgrade, but remember to have backups as a precautionary measure.
Copy & Paste in the LXC console
cat <<EOF >/etc/apt/sources.list
deb http://ftp.debian.org/debian bookworm main contrib
deb http://ftp.debian.org/debian bookworm-updates main contrib
deb http://security.debian.org/debian-security bookworm-security main contrib
EOF
apt-get update
DEBIAN_FRONTEND=noninteractive apt-get -o Dpkg::Options::="--force-confold" dist-upgrade -y
systemctl disable -q --now systemd-networkd-wait-online.service
rm -rf /usr/lib/python3.*/EXTERNALLY-MANAGED
apt-get autoremove --purge -y