The version that is needed by the Supervisor depends on the features it needs to work properly.
The current minimum supported version of Docker is:
However, the feature set changes and improves over time. Therefore, the minimal required version may change. When that happens, it will be communicated before we publish a version that requires you to upgrade Docker.
If you are running an older version of Home Assistant OS, update it.
If this is not Home Assistant OS, you need to manually update Docker on your host. For instructions on how to do that, check the official Docker documentation.