docs: Remove IRC reference.

For now point to #rust:mozilla.org. If that gets too much
bindgen-related traffic we can create an specific room for it.

Closes #2012

Author: emilio

CONTRIBUTING.md

@@ -2,7 +2,8 @@
 
 Hi! We'd love to have your contributions! If you want help or mentorship, reach
 out to us in a GitHub issue, or stop by
-[#servo on irc.mozilla.org](irc://irc.mozilla.org#servo) and introduce yourself.
+[#rust on chat.mozilla.org](https://chat.mozilla.org/#/room/#rust:mozilla.org)
+and introduce yourself.
 
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