maint: post-post-version-bump version bump to 6.0.0

My commit 3bbe1020 forgot
to update the news.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

docs/news.xml

@@ -42,7 +42,7 @@
      -->
 
 <libvirt>
-  <release version="v5.11.0" date="unreleased">
+  <release version="v6.0.0" date="unreleased">
     <section title="New features">
     </section>
     <section title="Improvements">