Use modern build status icon instead of "ball-"

The embeddable build status plugin still relies on the colored balls, as seen on https://ci.jenkins.io/job/Core/job/stapler/job/master/badge/ for example.

You can reference them via `ball-<size>`.

However, Jenkins does no longer use colored balls to display a build status. The legend displays the different states of build statuses: https://ci.jenkins.io/legend

Instead of manually inserting the colored ball PNGs, the embeddable-build-status plugin should make use of the API, to display the appropriate build status icons, you can find in other occasions in Jenkins.