Update IRC links

[philipzae]

With the move from freenode to libera chat confirmed, all IRC links on the website need to be updated.

[philipzae]

I've updated these links. https://xubuntu.org/irc/ https://xubuntu.org/help/ https://xubuntu.org/dev/ https://xubuntu.org/contribute/

[philipzae]

Updated some more links on these pages. https://xubuntu.org/contribute/artwork/ https://xubuntu.org/contribute/documentation/ https://xubuntu.org/contribute/marketing/ https://xubuntu.org/contribute/qa/

[bluesabre]

I did a quick google search (freenode site:xubuntu.org) and found two remaining instances on "current" content:

• https://contest.xubuntu.org/
• https://xubuntu.org/wp-content/uploads/2014/01/flyer_USletter_web.pdf