Test with latest workflow multibranch plugin

Author: MarkEWaite

pom.xml

@@ -197,7 +197,7 @@
     <dependency>
       <groupId>org.jenkins-ci.plugins.workflow</groupId>
       <artifactId>workflow-multibranch</artifactId>
-      <version>2.18</version>
+      <version>2.21</version>
       <scope>test</scope>
     </dependency>
     <dependency>