Replace public repo with releases repo

[MarkEWaite]

Replace public repo with releases repo

JFrog donates https://repo.jenkins-ci.org for the Jenkins project. They host it and pay all charges associated with that hosting.

They've asked us to reduce the https://repo.jenkins-ci.org outbound bandwidth use. One of the reductions is to retrieve released artifacts from their provider repositories instead of using https://repo.jenkins-ci.org/public as a cache of all other repositories.

This change removes https://repo.jenkins-ci.org/public from the list of repositories and replaces it with https://repo.jenkins-ci.org/releases/ for the Jenkins released artifacts and with other repositories for their artifacts.

Testing done

Automated tests pass locally on Java 11 with Linux. Rely on ci.jenkins.io to test Windows.

Submitter checklist

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