This fixes the problem for system packages (but not for imunify360):
https://ubuntuhandbook.org/index.php/2024/04/workaround-apt-warning-signature-key-uses-weak-algorithm/
tldr: Add a file
/etc/apt/apt.conf.d/99weakkey-warning
with
APT::Key::Assert-Pubkey-Algo ">=rsa2048,ed25519,ed448,dsa1024"...