Update Firebase UI Web version and host (#2796)

Co-authored-by: Leah E. Cole <[email protected]>
Co-authored-by: Bu Sun Kim <[email protected]>

Author: 3 people

appengine/standard_python37/building-an-app/building-an-app-3/templates/index.html

@@ -18,8 +18,8 @@
     }
   </script>
   <!-- [START gae_python37_auth_include_firebaseui] -->
-  <script src="https://cdn.firebase.com/libs/firebaseui/2.6.2/firebaseui.js"></script>
-  <link type="text/css" rel="stylesheet" href="https://cdn.firebase.com/libs/firebaseui/2.6.2/firebaseui.css">
+  <script src="https://www.gstatic.com/firebasejs/ui/4.4.0/firebase-ui-auth.js"></script>
+  <link type="text/css" rel="stylesheet" href="https://www.gstatic.com/firebasejs/ui/4.4.0/firebase-ui-auth.css" />
   <!-- [END gae_python37_auth_include_firebaseui] -->
   <script src="{{ url_for('static', filename='script.js') }}"></script>
   <link type="text/css" rel="stylesheet" href="{{ url_for('static', filename='style.css') }}">