[JENKINS-55085] - Use the main update center by default when running with Java 11

Reverts a custom experimental update center for Java 11 which was enabled to workaround the Java 11 compatibility issue in the Pipeline: Support plugin. See https://github.com/jenkinsci/jenkins/pull/3794 . Now Pipeline: Support plugin 3.0+ is available in the main update center, and a custom UC is nolonger required.