Automate updating the stable version

In order to reduce maintenance we need to automate the process of updating the official xwiki dockerhub image.

It involves:

• Downloading the WAR of the new version of XWiki and computing its sha256
• Update the https://github.com/xwiki-contrib/docker-xwiki/blob/master/build.gradle file and commit it
• Run gradle to modify the generated variants and commit them
• Ideally perform some minimal smoke tests
• Get the sha1 of the last commit and send a PR programmatically for https://github.com/docker-library/official-images/blob/master/library/xwiki
• Modify the release plan help with a new step to trigger this

More generally it means automating the steps from https://dev.xwiki.org/xwiki/bin/view/ReleasePlans/ReleasePlanHelp#HUpdateDocker so that can run something like:

./gradlew release

Note: The creation of a jira issue for the release might not be needed (similarly we don't create jira issues when releasing XWiki).