Migrate site.xml to new model (#9885)

core/src/site/site.xml

@@ -1,5 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <!-- This maven skin decorates the taglib documentation located at https://reports.jenkins.io/core-taglib/jelly-taglib-ref.html -->
-<project name="Jenkins">
+<site xmlns="http://maven.apache.org/SITE/2.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+      xsi:schemaLocation="http://maven.apache.org/SITE/2.0.0 https://maven.apache.org/xsd/site-2.0.0.xsd"
+      name="Jenkins">
     <body>
         <breadcrumbs>
             <item name="Homepage" href="https://jenkins.io/" target="_blank"/>
@@ -34,9 +37,7 @@
             ]]>
         </footer>
     </body>
-    <bannerLeft>
-        <name>Jenkins Taglib Documentation</name>
-    </bannerLeft>
+    <bannerLeft name="Jenkins Taglib Documentation"/>
     <skin>
         <groupId>org.apache.maven.skins</groupId>
         <artifactId>maven-fluido-skin</artifactId>
@@ -56,4 +57,4 @@
             <sideBarEnabled>false</sideBarEnabled>
         </fluidoSkin>
     </custom>
-</project>
+</site>