templates: substitute deprecated functions

templates/themes/sitemap/sitemap.xml

@@ -10,7 +10,7 @@
 		{% for thread in thread_list %}
 			<url>
 				<loc>{{ settings.url ~ (config.board_path | format(board)) ~ config.dir.res ~ link_for(thread) }}</loc>
-				<lastmod>{{ thread.lastmod | date('%Y-%m-%dT%H:%M:%S') }}{{ timezone() }}</lastmod>
+				<lastmod>{{ thread.lastmod | date('Y-m-d\\TH:i:s\Z') }}</lastmod>
 				<changefreq>{{ settings.changefreq }}</changefreq>
 			</url>
 		{% endfor %}