Move xom to testbom; update to 1.3.7

Core uses xom in testsuite/shared, but not elsewhere. Full WF uses xom in production code. So move the dependency management out of the root core pom and to testbom, which we created for this kind of situation.

The xom 1.2.10 we are using won't work with JDK 16 as it has some classes that subclass internal JDK classes, so move to 1.3.7, which eliminate that problem.

https://github.com/elharo/xom/releases/tag/v1.3.7