Upgrade XStream to improve Java 15+ compatibility

As a followup to #20 (closed), it turns out that getting the old XStream 1.4.8 version we have to work on Java 15 and newer would require far more tweaking than reasonable.

I have filed a CQ for the latest stable release as of date (1.4.19), and I am waiting for the result:

https://dev.eclipse.org/ipzilla/show_bug.cgi?id=24175

If it passes, we should be able to upgrade and automatically be compatible with Java 15. Java 16+ might still require some JVM flags.