mailing-lists: changed IRC link from freenode.net to libera.chat

This follows the similar change that we did in the backends manpage.

mailing-lists.html

@@ -109,9 +109,9 @@
       use the web interface to get it send to you again.
     </p>
 
-    <h2><a href="irc://irc.freenode.net/sane">SANE IRC Channel</a></h2>
+    <h2><a href="irc://irc.libera.chat/sane">SANE IRC Channel</a></h2>
     <p>
-      The SANE IRC channel #sane (irc.freenode.net) is intended for
+      The SANE IRC channel #sane (irc.libera.chat) is intended for
       discussions about SANE and for getting help. Please be patient, answers
       can take some time. Please read the documentation before asking. Tell us
       which scanner, which operating system and which version of SANE