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).

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