[JENKINS-51856] Add new milestones (#4450)

* [JENKINS-51856] Add new milestones

* Apply suggestions from code review

* [JENKINS-51856] Improve description of JOB_CONFIG_ADAPTED

* [JENKINS-51856] Add test for milestones

* [JENKINS-51856] Fix test error

* Apply suggestions from code review

Co-Authored-By: Oleg Nenashev <o.v.nenashev@gmail.com>

* Update core/src/main/java/hudson/init/impl/GroovyInitScript.java

Co-Authored-By: Tim Jacomb <t.jacomb@kainos.com>

Co-authored-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Co-authored-by: Tim Jacomb <t.jacomb@kainos.com>