New job/pipeline to prepare a release

This job will run only on master branch and is used to prepare the maintenance/reference branch before doing a release.

Define a new Jenkinsfile to:

• create the maintenance branch matching the current version (11.x, 12.x, ...)
• bump version on the `master` branch.

For instance, triggering this job with the current version being 11.1-SNAPSHOT will do:

• create a 11.x branch
• update the version on master to 12.0-SNAPSHOT