Investigate settings to publish build results fron Jenkins to GitHub

Main goal is to analyze what steps need to be taken to have the Jenkins
build results like compiler warnings and Maven problems be published to
the GitHub actions.

The initial step is just about how to get the results to GitHub. In a
later stage, those can the be further configured to have the build fail
if it would introduce additional warnings.

Jenkinsfile

@@ -98,6 +98,7 @@ BRANCH_NAME=${env.BRANCH_NAME}
   post {
     always {
       junit allowEmptyResults: true, testResults: '**/target/surefire-reports/*.xml'
+      discoverGitReferenceBuild referenceJob: 'build-classic/master'
       recordIssues publishAllIssues: true, tools: [java(), mavenConsole(), javaDoc()]
     }