chore: Prepare for icon removal from core (jenkinsci#192)

Author: NotMyFault

src/main/resources/org/jenkinsci/plugins/workflow/graph/FlowNode/sidepanel.jelly

@@ -26,8 +26,8 @@
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:l="/lib/layout" xmlns:t="/lib/hudson">
   <l:side-panel>
     <l:tasks>
-      <l:task icon="images/24x24/up.png" href="${rootURL}/${it.execution.owner.url}" title="${%Up}" contextMenu="false"/>
-      <l:task icon="images/24x24/search.png" href="${rootURL}/${it.url}" title="${%Status}" contextMenu="false"/>
+      <l:task icon="icon-up icon-md" href="${rootURL}/${it.execution.owner.url}" title="${%Up}" contextMenu="false"/>
+      <l:task icon="icon-search icon-md" href="${rootURL}/${it.url}" title="${%Status}" contextMenu="false"/>
       <st:include page="actions.jelly"/>
     </l:tasks>
   </l:side-panel>