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".
WIP action supercommand: registered with main
WIP action supercommand: removed wrong notes for defined
WIP CLI action supercommand: added common and common testing
WIP supercommand: add tabbedString method for outputs
TMP API: ActionUUIDs
TMP API: stub Actions(UUIDs) on action client
TMP API tweak to stub
WIP supercommand testing

Added stubbing features and Actions call.
WIP supercommand testing: tweaked package_test
TMP API Actions by UUID tweak: removed pointer type
WIP supercommand testing tweak: fix checkHelp