Mention official discord and Zulip, and remove IRC links

[jyn514]

Most of IRC has moved to either Discord or Zulip, but it still gets most of the front page.

[elasticdotventures]

I came here to say this.
https://matrix.org/blog/2020/03/03/moznet-irc-is-dead-long-live-mozilla-matrix

Rustacean Station Podcast on discord should be mentioned, as a noob to the Rust community I've found it indispensable to asynchronously 'catching up' on the history of RUST. 😆