deprecating the juju-backup (and juju-restore) CLI plugin

[Eric Snow]

The new "juju backups create" can be used instead of "juju backup" (a
CLI plugin).  So you would think we could deprecate and later remove
the old plugin. Unfortunately, "juju backups create" won't work with
1.20 or earlier API servers.  So it's not quite as simple as I'd hoped
as long as newer clients will be connecting to older servers.

What would be the best approach for deprecating the old backup plugin?
 I was going to simply gut it (it's a bash script) and stick in a
deprecation warning message and a call to "juju backups create
--download".  However, jam rightly pointed out to me that this may be
a sticky situation (as described above).  I'd really like to find a
way to eliminate the old backup implementation.  Thoughts?

-eric