Remove MultiPojoVersionMarshallTest

We need to disable MultiPojoVersionMarshallTest because Javassist (as of 3.20.0-GA) doesn't work with JDK 9.

There's no active work being done to support the use case shown by MultiPojoVersionMarshallTest. We should remove the test and instead write up a documentation section that shows how users could use externalizers to add multi version support.

So, this task contains a code part, and a documentation part. It should be resolve once both have been included.