bug#49004: Update IRC-related references to point to Libera.Chat

[Eli Zaretskii]

> From: Amin Bandali <bandali@gnu.org>
> Cc: eliz@gnu.org, larsi@gnus.org
> Date: Sun, 13 Jun 2021 11:01:35 -0400
> 
> I've prepared the following first revision against emacs-27.
> All comments/feedback welcome.
> 
> Eli, Lars, are you comfortable with the scope of the changes?

It's a large patch: is there anything here besides mechanical change
of the server address?

Btw, shouldn't we have some FAQ about the switch and its reasons?

Thanks.