[SAC] SAC IRC channel migrated to libera.chat

[Sandro Santilli]

As you may have heard, many Freenode staff members left
the now privately-owned network to fund Libera.Chat:

  https://en.wikipedia.org/wiki/Libera_Chat

Many free software projects already moved to the new network
and I think we should do the same, and quickly we're being
unregsitered forceably from Freenode (my "strk" account is
not active anymore, and I didn't intentionally remove it)

So I've removed the Freenode IRC bridge from the #sac:osgeo.org
matrix room and added a bridge to Libera IRC instead. It seems
to be working both ways just fine.

I've then updated the wiki page so that the link to the SAC
IRC channel points at libera.chat instead of freenode:

  https://wiki.osgeo.org/wiki/IRC#Infrastructure

And I've added the web UI for libera.chat in the relevant
section:

  https://wiki.osgeo.org/wiki/IRC#Web_based_IRC

A lot more on that page would need to be updated,
like community registration.

--strk;