[WFLY-19808] Move UndertowSubsystemModel to VERSION_15_0_0; add mixed…

[bstansberry]

… domain testing

We need a 15 version to:

• allow domain mode transformation, where a legacy HC could report it is running version 14. The WF 40 HC needs to know it is newer.
• so if someone doing a read-resource-description against two server running at 'default' stability, one WF 40 and one WF 39 doesn't see info saying the API version is 14 on both but one shows "allowed-request-attributes-pattern" as being an available attribute and the other doesn't.

I added testing against a version 14 server (EAP 8.1) to UndertowSubsystemTransformerTestCase.

[bstansberry]

@baranowb An explanation for this change...

We'll need to think a bit more about how to develop domain transformer stuff for things that are below default stability. It doesn't really work now and won't until wildfly/wildfly-core#6214 or something like it is done.

What you did in your earlier work on various undertow subsystem stuff at lower stability was good as it got the needed transformation logic in place in this class. But the version line ^^^ wasn't right, and there was no way to do it correctly, since no UndertowSubsystemModel.VERSION_15_0_0 existed at that time. I don't remember exactly what wildfly/wildfly-core#6214 does but I'm sure part of it adds a mechanism to differentiate between version 14 on an HC at default stability vs one at preview or community.

But right now we only support mixed domain between default stability HCs so we require transformation if the legacy HC is using version 14 or earlier.