Docs: Slack instead of irc (#8599)
Docs: Slack instead of irc (#8599)

#8562 notes that we don't use irc anymore. Replacing with
the current Slack info.
SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,duke8253/trafficserver,duke8253/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,pbchou/trafficserver,duke8253/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,pbchou/trafficserver,pbchou/trafficserver,pbchou/trafficserver,duke8253/trafficserver