Use the core sitemap.xml generation

Starting from concrete5 5.6.3 we can integrate the data generated by the standard sitemap.xml generator job, without the need of a duplicated job.

We can use add multilanguage data to the sitemap by extending the `on_sitemap_xml_addingpage` event.

During install or upgrade, the multilingual addon checks if concrete5 is >= 5.6.3, if the sitemap job is installed and enabled.
If so, it does not install the multilingual job and (if ENABLE_MULTILINGUAL_SITEMAPXML is set to true) extend the data from the standard job.
If concrete5 is < 5.6.3, of if the standard job is not available/enabled, nothing changes: we keep the old sitemap job up and running.