When I run "sudo apt update" on my system I get an error about the Dropbox key being stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg) and apt-key being deprecated. I searched google for an hour or so and tried multiple different solutions but can't find anything that works. It seems that the public key needed to verify the "http://linux.dropbox.com/debian/dists/sid/Release.gpg" key is not available. Any ideas?