[JENKINS-69153] Do not call deprecated h.singletonList (#71)

* Do not call deprecated h.singletonList

Jenkins 2.358 deprecated the `Functions.singletonList` method because
it is not normally needed in JEXL.  The `h.singletonList(it)` call in
this groovy code reports a null pointer exception in Jenkins 2.358 and
later.

When the `h.singletonList(it)` call is replaced with `it`, the null
pointer exception is not reported and the page loads.

* Pass a list

Co-authored-by: Jesse Glick <jglick@cloudbees.com>

Co-authored-by: Jesse Glick <jglick@cloudbees.com>

src/main/resources/org/jenkinsci/plugins/emailext_template/ExtEmailTemplateManagement/sidepanel.groovy

@@ -35,5 +35,5 @@ l.side_panel() {
       l.task(icon:"icon-next icon-md", href:"${rootURL}/emailexttemplates", title:_("Editable Email Templates"))
       l.task(icon:"icon-new-package icon-md", href:"addTemplate", title:_("Add New Template"))
     }
-    t.executors(computers:h.singletonList(it))
+    t.executors(computers: [it])
 }