Remove KXML2 library

[basil]

This PR removes the following JAR from the WAR:

• kxml2-2.3.0.jar

Core stopped depending on it in #7778. In that PR, I wrote:

I am not yet removing the kxml2 JAR file from the Jenkins WAR in this PR because I have not done the research to determine whether doing so would affect any plugins. If, once this change has been tested in several weekly releases and proven to be stable, there is a desire to remove this no-longer-needed JAR file, the removal can be explored at that time.

The above change has been shipping for a while now and we have not had a need to return to using KXML2 in core, so I am proceeding with the removal of this library from core. As you will see below, nothing of any relevance in the ecosystem is still using this functionality, and any stragglers are better off carrying this component themselves rather than relying on core for it.

I scanned the update centers for Jenkins and CloudBees and while I found some stuff using XPP3, I found nothing directly using KXML2.

Testing done

The PR build and update center scan should be sufficient. I also did a BOM run with this PR with no issues.

Proposed changelog entries

• Stop shipping net.sf.kxml:kxml2 because Jenkins no longer depends on it.

Proposed upgrade guidelines

N/A

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@mention

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[basil]

This change seems to break new XStream(), which uses the default driver (still XML Pull Parser v3), but not new XStream2(), which uses the StaX driver. In most cases we're already using (or should be using) new XStream2(), but there are still a handful of cases (mostly in tests) where we still need to use new XStream(). To get those uses to work with the removal of all three JARs mentioned above, they need to be changed to new XStream(XStream2.getDefaultDriver()). I'll explore how much work is needed to adapt the ecosystem to this change, and either do that work or reduce this scope of this PR by retaining at least one of the three JARs.

[basil]

While attempting to remove all three libraries (kxml2-2.3.0.jar, mxparser-1.2.2.jar, and xpp3-1.1.4c.jar) did lead me to file a few clean up PRs in various repositories to make plugin code more consistent with core (which was a nice benefit), I ended up giving up on removing mxparser after finding that it broke new XStream(), which would break third-party library code that happens to use XStream (independent of Jenkins plugins or Jenkins core itself) and I ended up giving up on removing xpp3 after finding some proprietary plugins that directly imported XppDriver. So the scope of this PR is now just removing kxml2 which should be completely safe to do today. Therefore I am removing the on-hold label.

[amuniz]

ended up giving up on removing xpp3 after finding some proprietary plugins that directly imported XppDriver

I think its usage in that particular proprietary plugin (operations-center-context) can be just removed (as long as there is an XStream constructor not requiring an instance of it).

[basil]

I think its usage in that particular proprietary plugin […] can be just removed

I am sure that it can be. But the fact that it has not been is the reason that I have given up on this removal and reduced the scope of this PR.

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!