Undertow subsystem model for WF27 should be 12.0.0, not 13.0.0.

This commit: https://github.com/wildfly/wildfly/commit/a1f9ad78f6c960e824a2ca007a40593706c2d060

incremented the model version from 11.0.0 -> 13.0.0 completely skipping version 12.  This is going to cause confusion and should be addressed before WF27 is released.