[poppler] Moved IRC to libera.chat

[Albert Astals Cid]

The freenode staff deleted the #poppler channel and all the registered users, so I've edited the webpage to now say libera.chat instead of freenode.net

I'm sorry I didn't get to discuss this before implementing it, but given the super bad behaviour by the freenode staff the action needed to be taken immediately.

If someone has any reservations about libera.chat and would prefer any other network please let's talk about it :)

Cheers,
  Albert