Remove references to IRC from CONTRIBUTING.md

[BenjaminAmos]

Description

I noticed that the CONTRIBUTING.md document still references IRC, which I do not believe has been in-use for a while. As far as I know, activity is almost exclusively on Discord these days. Is there even still a #destination-sol channel on freenode anymore?

I've also changed it so that support requests are pointed towards Discord, rather than the forum. This can be changed if necessary but the forum's also been inactive for a while.