[JENKINS-75008] Replace outdated poll SCM icon with play symbol (#48)

The play symbol is used by the "Scan repository" button for multibranch
Pipelines and is used by the "Build Now" button for all jobs.  Make the
poll SCM plugin consistent with other usages by switching to the play
icon.

https://issues.jenkins.io/browse/JENKINS-75008 is the bug report

Testing done:

Created a freestyle project that uses a git repository and enabled
polling in the job definition so that the "Poll now" button was visible in
the sidepanel.  Confirmed that the button in the sidepanel was correct.
Confirmed that clicking the button in the side panel performed a poll
of the repository.

Ran the automated tests and confirmed that they pass with the changes.

src/main/java/org/jenkinsci/plugins/pollscm/PollNowAction.java

@@ -66,7 +66,7 @@ public SCMTriggerItem getOwner() {
     }
 
     public String getIconFileName() {
-        return getACL().hasPermission(POLL) ? "/plugin/pollscm/images/24x24/clipboard-play.png" : null;
+        return getACL().hasPermission(POLL) ? "symbol-play" : null;
     }
 
     private ACL getACL() {

src/test/java/org/jenkinsci/plugins/pollscm/PollNowActionTest.java

@@ -60,7 +60,7 @@ void getIconFileNameWithPermission() {
             jenkinsMockedStatic.when(() -> Jenkins.getInstance()).thenReturn(jenkins);
             when(acl.hasPermission(PollNowAction.POLL)).thenReturn(true);
             String iconName = mockPollNowAction.getIconFileName();
-            assertEquals("/plugin/pollscm/images/24x24/clipboard-play.png", iconName);
+            assertEquals("symbol-play", iconName);
         }
     }