What I have found out so far:
If one would use wildfly 12, it is possible to use JSF 2.3 by enabling ee8.preview.mode, since then you would have Java EE 8, which comes with JSF 2.3.
The currently newest release of wildfly-swarm (thorntail) 2018.5.0 uses wildfly 11, which comes with Java EE 7, and comes with JSF 2.2.13.
So my question is, how can I enable JSF 2.3 using wildfly-swarm/thorntail? Or if this is impossible, when will wildfly-swarm/thorntail be bundled with wildfly 12, so that it is also bundled with JSF 2.3?