apt-key has been deprecated. Here’s a quick one-liner to fix the annoying message during apt-get update . Ideally, you’d want to pluck out each key using apt-key list then apt-key export <id> placing each key in it’s own file under /etc/apt/trusted.gpg.d.
sudo apt-key --keyring /etc/apt/trusted.gpg exportall | \
sudo tee /etc/apt/trusted.gpg.d/all_keys.asc