Fix apt key warning for Helm repository
Remove this warning:
Repository update W: https://baltocdn.com/helm/stable/debian/dists/all/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
Fix found in Helm documentation : https://helm.sh/docs/intro/install/#from-apt-debianubuntu