Gut the backup plugin, but keep it around for backward-compatibility.
Use the old plugin code for 1.20.
Add a deprecation warning message.
Add a TODO comment about removing the plugin.
Improve the visibility of the divider comment.
Skip new backups for 1.21 as well.
2sp -> 1sp.
Merge pull request #1003 from ericsnowcurrently/backups-kill-plugin

Deprecate the backup CLI plugin.

We must keep "juju backup" around for backward-compatibility.  However, for everything but 1.20 (when "juju backup" was introduced) and 1.21 we can simply call "juju backups create --download".