Support JDK versions newer than JDK8

[markaren]

osp-validator will only work on JDK8 systems due to the dependency on JAXB. This might be hard to swallow for many.
However, JAXB can be used on newer JDKs if the dependency is explicitly listed.

    implementation group: 'javax.xml.bind', name: 'jaxb-api', version: '2.3.1'
    implementation group: 'org.glassfish.jaxb', name: 'jaxb-core', version: '2.3.0.1'
    implementation group: 'org.glassfish.jaxb', name: 'jaxb-runtime', version: '2.3.3'

Developers that want to autogenerate sources from xsd still need JDK8, but users may use any JVM version.