Bump XStream from 1.4.17 to 1.4.18 (#5685)

* Bump xstream from 1.4.17 to 1.4.18

Bumps [xstream](https://github.com/x-stream/xstream) from 1.4.17 to 1.4.18.
- [Release notes](https://github.com/x-stream/xstream/releases)
- [Commits](https://github.com/x-stream/xstream/commits

)

---
updated-dependencies:
- dependency-name: com.thoughtworks.xstream:xstream
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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* Adapting `hudson.util.RobustReflectionConverterTest.ifWorkaroundNeeded`.

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>