Planned breaking change for plugin builds: require <repositories> / <pluginRepositories>

[Jesse Glick]

All plugin POMs (and other POMs in the Jenkins org) are expected to specify

<repositories>
  <repository>
    <id>repo.jenkins-ci.org</id>
    <url>https://repo.jenkins-ci.org/public/</url>
  </repository>
</repositories>
<pluginRepositories>
  <pluginRepository>
    <id>repo.jenkins-ci.org</id>
    <url>https://repo.jenkins-ci.org/public/</url>
  </pluginRepository>
</pluginRepositories>

so that they can build against our Artifactory server without special settings. If https://github.com/jenkins-infra/pipeline-library/pull/216 is merged, this will be enforced on ci.jenkins.io builds—today the CI server is lenient. If you see build failures relating to being unable to download Jenkins-related artifacts, please check that your pom.xml includes the above snippet.