Switch from deprecated (since Hugo 0.124) .Site.LastChange to the new Site.Lastmod

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

themes/apertis/layouts/partials/footer.html

@@ -31,7 +31,7 @@
         <p>
           {{ .Site.Author.name }}
           &nbsp;&bull;&nbsp;
-          {{ .Site.LastChange.Format "2006" }}
+          {{ .Site.Lastmod.Format "2006" }}
 
           {{ if .Site.Title }}
             &nbsp;&bull;&nbsp;