Recover the public key

wget https://faster.in2p3.fr/distribution/fasterv2/faster-packages.pub

Install the public key in /etc/apt/trusted.gpg.d/faster.gpg

cat faster-packages.pub | sudo gpg --dearmour -o /etc/apt/trusted.gpg.d/faster.gpg

Configure the FASTER apt source file in /etc/apt/sources.list.d/faster.sources

Enabled: yes
Types: deb
URIs: https://faster.in2p3.fr/distribution/fasterv2/ubuntu/
Suites: noble
Components: non-free
Signed-By: /etc/apt/trusted.gpg.d/faster.gpg

Update your package sources list

sudo apt update