The latest Flutter requires intl 0.19.0 and now every app/package
depending on us fails to build with the latest Flutter because we were
explicitly requiring 0.18.0
Minor upgrades should be safe anyway, we only care for major versions
upgrade, so loosen the requirements a bit to support anything from
0.18.0 up to (but not including) 1.0.0
Now it is no longer possible to break the widget by providing a null value to the disabledPickers property.
Making the property to a non-nullable list would remove more complexity, but introduces breaking changes.