Jenkins: Add a global build timeout

Prepares migration to oss.cloudogu.com.
There, we're recognized some builds that hang for hours after some
errors.

Jenkinsfile

@@ -11,9 +11,11 @@ node() { // No specific label
 
   properties([
     // Keep only the last 10 build to preserve space
-    buildDiscarder(logRotator(numToKeepStr: '10')),
+    buildDiscarder(logRotator(numToKeepStr: '10'))
   ])
 
+  timeout(activity: true, time: 20, unit: 'MINUTES') {
+
     catchError {
 
       Maven mvn = setupMavenBuild()
@@ -51,6 +53,7 @@ node() { // No specific label
     warnings consoleParsers: [[parserName: 'Maven']], canRunOnFailed: true
 
     mailIfStatusChanged(commitAuthorEmail)
+  }
 }
 
 String mainBranch