Enhance rollback to attempt to find a version when one is not specified

[mike-weiner]

Closes https://github.com/basecamp/kamal/issues/1658.

This PR attempts to further reduce downtime when something goes wrong and a rollback is required. kamal rollback [VERSION] still exists as-is. kamal rollback (no version specified) is also now an option and attempts to automatically find a version present on all hosts/roles and initiate a rollback to that version.