Replace keepLongStdio property with stdioRetention

[C0urante]

This ports upstream changes from the JUnit plugin that replaced the keepLongStdio property with stdioRetention in https://github.com/jenkinsci/junit-plugin/pull/601. Without this change, this plugin is incompatible with newer releases of the JUnit plugin.

Testing done

I tweaked the POM to use the latest release of the JUnit plugin, then ran existing unit tests. As reported elsewhere, the RealtimeJUnitStepTest::ui case failed. After these changes, that test succeeded, and I was able to verify that the UI contains the correct values for the property in the dropdown form. For good measure, I also added another round trip config test with a new value for the stdioRetention property.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[C0urante]

CI is failing right now because an older version of the JUnit plugin is still used on this branch. Should pass when the dependency is updated.

[alecharp]

Please, use the dependency management to fix the unit version coming from the BOM. Once the bom is released and updated here, that could be removed.

[C0urante]

@alecharp thanks for the guidance on resolving the dependency misalignment. I gave it several tries using different variants of dependencyManagement and even a version property, but all of those still got overridden (see the CI failures on recent commits). I've gotten things to work with an explicit dependency element. I know it's a bit crass so please let me know if there's a cleaner way to go about this.

[timja]

Thanks!