Move Jenkins service to Core

As discussed at !37797 (diffs, comment 386307481) the Jenkins service at this point is "just a convenience wrapper. All functionality works with a simple webhook, including the CI status reporting back from Jenkins." Let's move Jenkins service to Core.

the only thing it appears this integration does is run a webhook: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/project_services/jenkins_service.rb#L35 which can be manually configured as detailed at https://github.com/jenkinsci/gitlab-plugin#configuring-per-project-authentication.

I don't see a ton of value in having it be EE-only anymore. That's likely a holdover from when the original Jenkins integration had quite a bit of logic in it. Now the Jenkins plugin itself down the heavy lifting.