chore: Prepare for icon removal from core (#58)

Author: NotMyFault

src/main/resources/io/jenkins/plugins/pipelinegraphview/PipelineGraphViewAction/sidepanel.jelly

@@ -5,7 +5,7 @@
     <l:side-panel>
         <l:tasks>
             <j:set var="buildUrl" value="${h.decompose(request)}"/>
-            <l:task icon="images/24x24/up.png" href="${buildUrl.baseUrl}/" title="${%Back to Build}" contextMenu="false"/>
+            <l:task icon="icon-up icon-md" href="${buildUrl.baseUrl}/" title="${%Back to Build}" contextMenu="false"/>
         </l:tasks>
     </l:side-panel>
 </j:jelly>

src/main/resources/io/jenkins/plugins/pipelinegraphview/consoleview/PipelineConsoleViewAction/sidepanel.jelly

@@ -5,7 +5,7 @@
     <l:side-panel>
         <l:tasks>
             <j:set var="buildUrl" value="${h.decompose(request)}"/>
-            <l:task icon="images/24x24/up.png" href="${buildUrl.baseUrl}/" title="${%Back to Build}" contextMenu="false"/>
+            <l:task icon="icon-up icon-md" href="${buildUrl.baseUrl}/" title="${%Back to Build}" contextMenu="false"/>
         </l:tasks>
     </l:side-panel>
 </j:jelly>

src/main/resources/io/jenkins/plugins/pipelinegraphview/multipipelinegraphview/MultiPipelineGraphViewAction/sidepanel.jelly

@@ -5,7 +5,7 @@
     <l:side-panel>
         <l:tasks>
             <j:set var="buildUrl" value="${h.decompose(request)}"/>
-            <l:task icon="images/24x24/up.png" href="${buildUrl.baseUrl}/" title="${%Back to Build}" contextMenu="false"/>
+            <l:task icon="icon-up icon-md" href="${buildUrl.baseUrl}/" title="${%Back to Build}" contextMenu="false"/>
         </l:tasks>
     </l:side-panel>
 </j:jelly>