Starting with Java 9+, JAXB has been removed from the JDK. So it's a good thing to try and break/remove the dependency against from plugins who do not really use JAXB for its core feature (object de/serialization).
(only used in tests for this plugin, so nothing critical)
Acceptance criteria:
Created PR in the plugin Github project: https://github.com/jenkinsci/timestamper-plugin/pull/26