Merge pull request juju#16912 from hpidcock/jujud-controller-4

juju#16912

With moving to the jujud-controller snap and a new process for development build/upgrade process, it makes sense to bifurcate the upgrade-model and upgrade-controller commands because they will fundamentally serve two different purposes into the future.

Since `--build-agent` was only supported on controller models, this change simplifies the upgrade model command greatly.

Since `upgrade-controller` command used to invoke the `upgrade-model` command in a funky way, by bifurcating, we actually simplify both commands.

## QA steps

- Bootstrap controller
- Upgrade controller
- Upgrade model
- Use `--build-agent` on the `upgrade-controller` command too.

## Documentation changes

- Can no longer upgrade a controller via upgrade-model.

## Links

**Jira card:** JUJU-5439

Author: jujubot

cmd/juju/commands/main.go

@@ -399,7 +399,7 @@ func registerCommands(r commandRegistry) {
 	r.Register(model.NewModelGetConstraintsCommand())
 	r.Register(model.NewModelSetConstraintsCommand())
 	r.Register(newSyncAgentBinaryCommand())
-	r.Register(newUpgradeJujuCommand())
+	r.Register(newUpgradeModelCommand())
 	r.Register(newUpgradeControllerCommand())
 	r.Register(application.NewRefreshCommand())
 	r.Register(application.NewSetApplicationBaseCommand())