Update irc links to libera.chat.

Channels have moved to irc.libera.chat.

Libera doesn't seem to have a self-hosted web client, so I've
just removed those links. mibbet does work, but the interface
is confusing; I think people are better off with just the
`irc:` intent for now.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>