maintainers: Update freedesktop.org IRC channels

maintainers: Update freedesktop.org IRC channels

Like many free software projects, freedesktop.org issued a non-binding
recommendation for projects to migrate from Freenode to OFTC [1]. As
such, freedesktop.org entries in the MAINTAINERS file are out-of-date as
the respective channels have moved. Update the file to point to the
right network.

v2: Correct typo in commit message pointed out by Lukas and Jonathan.
Add Hector's ack.

v3: Add acks.

[1] https://lists.freedesktop.org/archives/dri-devel/2021-May/307605.html

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Hector Martin <marcan@marcan.st>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210725150007.2818-2-alyssa@rosenzweig.io