Ban JUnit 4 imports (#142)

All tests in the plugin are using JUnit 5.

Author: MarkEWaite

pom.xml

@@ -41,6 +41,7 @@
     <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>
     <hpi.compatibleSinceVersion>3.2</hpi.compatibleSinceVersion>
     <spotless.check.skip>false</spotless.check.skip>
+    <ban-junit4-imports.skip>false</ban-junit4-imports.skip>
   </properties>
 
   <dependencyManagement>