Update copyright year and contributor information

Author: fmaussion

_config.yml

@@ -27,7 +27,7 @@ html:
     <p>
     These notebooks are licensed under a <a href="https://github.com/OGGM/tutorials/blob/master/LICENSE.txt" target="_blank">BSD-3-Clause license</a>.
     <br>
-    &copy; Copyright 2014-2021.
+    &copy; Copyright 2014-2026 OGGM Contributors. <a href="https://community.oggm.org/legal.html" target="_blank" rel="noopener noreferrer">Legal & Privacy</a>
     </p>
 sphinx:
   config: