tpl: fix incorrect lastBuildDate

Set the `<lastBuildDate>` field to be the most recent Lastmod date
of all the posts in the current selection.

Fixes gohugoio#11600

tpl/tplimpl/embedded/templates/_default/rss.xml

@@ -43,7 +43,7 @@
     <managingEditor>{{.}}{{ with $authorName }} ({{ . }}){{ end }}</managingEditor>{{ end }}{{ with $authorEmail }}
     <webMaster>{{ . }}{{ with $authorName }} ({{ . }}){{ end }}</webMaster>{{ end }}{{ with .Site.Copyright }}
     <copyright>{{ . }}</copyright>{{ end }}{{ if not .Date.IsZero }}
-    <lastBuildDate>{{ .Date.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
+    <lastBuildDate>{{ (index $pages.ByLastmod.Reverse 0).Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
     {{- with .OutputFormats.Get "RSS" }}
     {{ printf "<atom:link href=%q rel=\"self\" type=%q />" .Permalink .MediaType | safeHTML }}
     {{- end }}