Use core default driver for XStream

[basil]

The XStream default driver (based on XML Pull Parser 3rd edition) is one that core no longer prefers or uses for various reasons, including bugs parsing emoji in modern XML files. Instead core now uses the Streaming API for XML (StAX) that is built into modern versions of the Java Platform runtime environment. I have been working to slowly remove the old XPP3 parser from core, but I discovered that this plugin still had a hard-coded reference to it in one place and was broken when I removed the old driver from core. To avoid this problem I am instead having this plugin use core's default driver, which in newer versions is StAX.

Testing done

I ran this plugin's tests against a core with XPP3 removed and observed they started failing. After this PR and jenkinsci/nested-view-plugin#43 they now pass.

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[jamietanna]

@basil want me to merge? Or you happy to?

[basil]

I am an org admin but not a maintainer of this plugin, and org admins generally do not merge changes for plugins they are not maintaining (even though they have the permissions to) as part of our social contract, unless some exception presents itself.

[jamietanna]

All good, thought I'd ask! Would it be handy to have it released sooner than later?

[basil]

Yes, it would be nice to get this cleaned up and released so that core can eventually remove the outdated driver once all consumers have been eliminated.

[jamietanna]

@basil all released now, thanks for your patience 🚀

[basil]

Thank you very much!