Update documentation informing newcomers of NULL IRC SERVER

"...that the IRC Mozilla Server no longer EXISTS!"

I recently started into Rust and was led to believe there existed a #rust channel.
I have since found out otherwise after about 40mins of googling and searching.

https://wiki.mozilla.org/IRC I see this page is outdated. It should be removed though....
https://developer.mozilla.org/en-US/docs/Mozilla/Getting_started_with_IRC Why does this even exist?

Mozilla only recently shut down its IRC server (about 2 weeks ago), it might be understandable that not all of their docs are updated yet.
The Wiki is, and it has an explicit message that the IRC site is outdated, but kept for historical purposes.
MDN is lacking any of that messaging, I'll noted it down and will bring that up on Monday when back at work.

The Rust project itself moved away from IRC back in 2018 already, see this thread and a later announcement blog post.
If the project still lists IRC as one of the available online venues, please do let us know and we can change that information.

Both of these pages have nothing to do with the Rust project and are out of our control. You should bring this up with Mozilla, though I don’t know enough about how to do so to point you in the right direction.

I edited the MDN page and relayed information further to maybe delete it.
Also trying to make the message on the wiki page more prominent