Document our migration of irc to libera.chat

Discussed on dev@
https://mail-archives.apache.org/mod_mbox/subversion-dev/202105.mbox/%3C8FA19AD8-FEC9-4CE6-84EB-ECDCD68801AA%40apache.org%3E

* README
  s/freenode\.net/libera\.chat/


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1890231 13f79535-47bb-0310-9956-ffa450edef68

Author: dsahlberg-apache-org

README

@@ -49,7 +49,7 @@ III.  PARTICIPATING IN THE SUBVERSION COMMUNITY
       It describes Subversion coding and log message standards, as well
       as how to join discussion lists.
 
-      Talk on IRC with developers:  irc.freenode.net, channel #svn-dev.
+      Talk on IRC with developers:  irc.libera.chat, channel #svn-dev.
 
       Read the FAQ:  https://subversion.apache.org/faq.html