Update IRC-related references to point to Libera.Chat

Update IRC-related references to point to Libera.Chat

Per GNU and FSF's announcements [0, 1] of moving official IRC channels
to the Libera.Chat IRC network, as well as several Emacs-related
channels following suit [2], update IRC-related references to reflect
the migration.

[0]: https://lists.gnu.org/archive/html/info-gnu/2021-06/msg00005.html
[1]: https://lists.gnu.org/archive/html/info-gnu/2021-06/msg00007.html
[2]: https://lists.gnu.org/archive/html/info-gnu-emacs/2021-06/msg00000.html