Change Freenode IRC link to Libera.Chat

[Nezteb]

tl;dr Freenode was taken over and most staff moved to Libera.Chat.

• https://en.wikipedia.org/wiki/Freenode#Ownership_change_and_conflict
• https://arstechnica.com/gadgets/2021/05/freenode-irc-has-been-taken-over-by-the-crown-prince-of-korea/
• https://www.jeffgeerling.com/blog/2021/freenode-dead-long-live-irc

[JanWielemaker]

Thanks. Will be on the site with the next release.