update FOSS4G link

Author: jmckenna

_templates/indexsidebar.html

@@ -16,7 +16,7 @@ <h3>Navigation</h3>
 <p>MapServer is a project of the <a href="https://www.osgeo.org/" target="_blank">
 Open Source Geospatial Foundation</a>.</p>
 <p>
-<a href="https://www.osgeo.jp/foss4g-2026-hiroshima" target="_blank">
+<a href="https://2026.foss4g.org/" target="_blank">
     <img src="{{ pathto("_static/foss4g2026.png", 1) }}" alt="FOSS4G" border="0px; text-decoration:none" />
 </a>
 </p>