[JENKINS-72067] Maintain just one XStream2 for changelog parsing

[jglick]

Complementing jenkinsci/jenkins#8529. Untested; the parent POM here is so old I am unable to even compile the plugin.

Regardless of core version, this change seems desirable since constructing an XStream instance and warming up its converters is potentially expensive.

[basil]

the parent POM here is so old I am unable to even compile the plugin

An example of

This unfinished work becomes technical debt throughout the ecosystem and interferes with unrelated projects, exploding the scope of otherwise simple projects from "implement an RFE" to "implement an RFE and deal with an unbounded number of unfinished and often poorly documented migrations

from jenkinsci/archetypes#650.