Replace IRC with Discord

Discord has more users. IRC traffic has ground to a halt.

No real objection to
https://lists.ozlabs.org/pipermail/openbmc/2020-November/023984.html



Removed IRC and Riot from the README and replaced IRC in other places.

Change-Id: I7124b11320f6c31e9e0a2aead35274690ff0f908
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>