Flatten <repositories> part of deployed pom when using JEP-229

[Vlatombe]

Counterpart of jenkinsci/plugin-pom#802,
workarounds mojohaus/flatten-maven-plugin#362

Testing done

Updated pom of stapler with SNAPSHOT of this project

Ran

mvn -Pquick-build -Dset.changelist -Dignore.dirt verify

Verified that **/target/stapler-*-<version>.pom no longer has a <repositories> section.

### 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
- [ ] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

Thanks for the PR!