Change IRC channel to LiberaChat in README (#16279)

Freenode was taken over by a different entity, and former Freenode
staff created LiberaChat.

Short summary of the situation: https://blog.bofh.it/debian/id_461

With more details:

* https://gist.github.com/joepie91/df80d8d36cd9d1bde46ba018af497409
* https://gist.github.com/aaronmdjones/1a9a93ded5b7d162c3f58bdd66b8f491

README.md

@@ -82,7 +82,7 @@ Mastodon is **free, open-source software** licensed under **AGPLv3**.
 
 You can open issues for bugs you've found or features you think are missing. You can also submit pull requests to this repository, or submit translations using Crowdin. To get started, take a look at [CONTRIBUTING.md](CONTRIBUTING.md). If your contributions are accepted into Mastodon, you can request to be paid through [our OpenCollective](https://opencollective.com/mastodon).
 
-**IRC channel**: #mastodon on irc.freenode.net
+**IRC channel**: #mastodon on irc.libera.chat
 
 ## License