[Checkout] Rename increment_id for CustomerOrder type

[YevSent]

Summary

The CustomerOrder type has increment_id field which represents order number on the storefront. The backend service contracts use this field as is but for the storefront API doesn't have business logic for it and the most usage - show to customer the number of the order.

Acceptance Criteria

• Deprecate increment_id in CustomerOrder type
• Introduce order_number: String! in CustomerOrder type
• Cover with tests

[lenaorobei]

AC:

• if this field was already released, it is marked as deprecated instead of removing
• covered with api-functional tests

[okorshenko]

@strell \Magento\GraphQl\Sales\OrdersTest

Example: XxXgeoXxX@2c4adbf