Docs: Slack instead of irc (#8599)

#8562 notes that we don't use irc anymore. Replacing with
the current Slack info.

(cherry picked from commit 3dd78a7)

Author: mlibbey

doc/preface/index.en.rst

@@ -118,12 +118,7 @@ Developer List
     Send an email to ``dev-subscribe@trafficserver.apache.org`` to join the
     list.
 
-Internet Relay Chat (IRC)
--------------------------
-
-The ``#traffic-server`` channel on ``irc.freenode.net`` is the official IRC
-resource for the |TS| project, and boasts active discussions.
-
-Community Forums
-----------------
+Slack
+-----
 
+The `traffic-server <https://the-asf.slack.com/archives/CHQ1FJ9EG>`_ channel on the |ASF| `Slack <https://infra.apache.org/slack.html>`_ is the official live chat resource for the |TS| project, and boasts active discussions.