docs: move the IRC channel away from Freenode

docs: move the IRC channel away from Freenode

Due to the recent events at Frenode [0], the channel has become a bit
unreliable (much spammed), and users have started to move away already,
as quite a few other projects have moved their IRC presence away from
Freenode.

There are a few alternatives. The first to spring to mind, is the new
Libera.Chat network [1], managed by the previous Freenode staff, so we
could expect quite a good experience there. However, it is a very young
network. The second well known alternative is the long-established OFTC,
which has been very reliable in its 20 years of existence.

So, let's move to OFTC, just because it has a track-record of robustness
(which Libera.Chat still has to build, for being young).

Note: there are a lot of other IRC networks, some very good too, but we
probably would be much off-topic on most of them.

[0] https://lwn.net/Articles/856543/
[1] https://libera.chat/
[2] https://www.oftc.net/

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Matthew Weber <matthew.weber@collins.com>
Acked-by: Heiko Thiery <heiko.thiery@gmail.com>
Acked-By: Vincent Fazio <vfazio@xes-inc.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>