Fix warning about trusted.gpg when updating from apt mirror

[palpant-dbx]

The trusted.gpg keyring (and apt-key) is deprecated, which leads to a warning whenever you apt update from the mirror.

This switches to the recommended alternative, which is to place our public key in /etc/apt/keyrings and then reference it in the repo mirror configuration.

Tested by building a deb package and then:

# dpkg -i dropbox_2022.12.05_amd64.deb
# apt update
# apt remove dropbox
# apt install dropbox

and verified there are no warnings or errors

[palpant-dbx]

A fix for the fedora canary error is included in #119 and is unrelated to this change (just a new Fedora version that needs to be added).