Update sitemap protocol implementation to version 0.9

Currently the sitemap.xml is of version 0.84, as seen in the source of the sitemap.xml generated by Liferay. e.g.: http://localhost:8080/sitemap.xml?groupId=10180&privateLayout=false

<urlset xmlns="http://www.google.com/schemas/sitemap/0.84">

According to Google Webmaster tools, the current version is 0.9 and previous versions are no longer supported:

Google adheres to Sitemap Protocol 0.9 as defined by sitemaps.org. The Sitemap protocol uses an XML schema to define the elements and attributes that can appear in your Sitemap file. Previous versions of the schema (for example, Sitemap Protocol 0.84) are now deprecated.

Sitemaps.org also shows 0.9 as the latest version.