Use rclone backend for duplicity cloud storage

With the deprecation of the pydrive Duplicity backend, rather than change to the new Google-specific backend, let's just unify our cloud storage solution between Restic and Duplicity.

This should also make it much easier for us to allow advanced users to use weird custom cloud locations if we want to expose that.

As part of this, we need to bump the minimum duplicity version to at least 0.8.12, but while we're doing that, we might as well bump to 2.0.0 to avoid having to handle pre-2.x CLI option changes.