Add a downgrade command, so you can reverse the upgrade process and go
back to Kamal 1.9. This replaces kamal-proxy and reboots all the
accessories.
This gives an upgrade and downgrade path:
Upgrade:
1. Upgrade config to be Kamal 2 compatible + use kamal 2.0
2. Run `kamal upgrade`
Downgrade:
1. Switch back to previous config + use kamal 1.9
2. Run `kamal downgrade`
You can set `--rolling` to downgrade one host at a time.
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