chore: Prepare for icon removal from core

Author: NotMyFault

Diff for: src/main/java/hudson/plugins/nested_view/NestedView.java

@@ -296,7 +296,7 @@ void setOwner(ViewGroup owner) {
 
     public String getImage(View w) {
         if (w instanceof  NestedView) {
-            return "folder.gif";
+            return "folder.png";
         } else {
             return "clipboard.png";
         }

Diff for: src/main/resources/hudson/plugins/nested_view/NestedView/sidepanel2.jelly

@@ -23,7 +23,7 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 -->
 <?jelly escape-by-default='true'?>
-<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:s="/lib/form">
-  <l:task icon="images/32x32/folder.gif" href="${rootURL}/${it.viewUrl}newView" title="${%New View}"
+<j:jelly xmlns:j="jelly:core" xmlns:l="/lib/layout">
+  <l:task icon="icon-folder icon-md" href="${rootURL}/${it.viewUrl}newView" title="${%New View}"
    permission="${it.CONFIGURE}" />
 </j:jelly>