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