bin/cake bake migration_snapshot should use `change` instead of `up` and `down`

[geoidesic]

According to the docs, change is the new preferred format. So Bake should produce this format for migrations. Currently it produces up and down.

[dereuromark]

Change has one major disadvantage:
It cannot be reversed (rollback) for a few topics.

Maybe there could be an option added? --change
To use that instead of up/down?