Fix CII rule for BT-31to only accept VAT in future

Created by: rkottmann

CII syntax binding demands ram:ID/@schemeID="VA" for BT-31. CII schematron demands ram:ID/@schemeID="VAT" for BT-31. Until this conflict has been resolved, we allow for both alternatives in BR-DE-16.

However, this is a breaking change and we need to be in sync with CEN developemnt.