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".
juju-run: adding relation-id and remote-unit support
juju-run: adding and updating tests
juju-run: added --no-remote-unit for explicit executions against relations with no remote unit
juju-run: ParseRemoteUnit fixes and tests
juju-run: adding additional conditions to the parseRemoteUnit test, review updates
jujuc: make relation flags match juju-run
juju-run: updates based on peer review
extended jujuc / juju-run support the same argument flags
renamed --no-remote-unit to --skip-remote-unit-check
changed --relation-id to --relation, added parsing of string inputs
added more testing
juju-run: add --relation to jujuc relation commands
juju-run: assign result to satisfy the client.Call API
juju-run: updated to use errors package
juju-run: peer review updates and more tests
juju-run: ParseRemoteUnit ambiguous testing