backup.yml: Dropbox now supports U2F!
backup.yml: Dropbox now supports U2F!

This change updates the entry for Dropbox to reflect their recently added support for U2F tokens.

https://www.dropbox.com/en/help/363#2fa-security-keys
trim108/trim108.github.io,kbuley/dongleauth,kbuley/dongleauth,trim108/trim108.github.io,kbuley/dongleauth,Nitrokey/dongleauth,Nitrokey/dongleauth,Nitrokey/dongleauth,trim108/trim108.github.io