Change IRC pointers to point to Libera.Chat

freenode experienced a hostile takeover:
https://gist.github.com/joepie91/df80d8d36cd9d1bde46ba018af497409/
https://gist.github.com/aaronmdjones/1a9a93ded5b7d162c3f58bdd66b8f491
etc.

Essentially all of freenode staff founded/moved over to Libera.Chat:
https://libera.chat/news/welcome-to-libera-chat

whipper’s official IRC presence will also be on Libera.Chat going
forward. freenode channel will exist for a bit longer, but will
eventually get locked.

AFAIK, there is no Matrix bridge yet(?), so removing the Matrix bit for
now. Will be added back in later when a bridge is established.

Resolves https://github.com/whipper-team/whipper/issues/540

Signed-off-by: Frederik “Freso” S. Olesen <freso.dk@gmail.com>

README.md

@@ -445,10 +445,9 @@ And to all the [contributors](https://github.com/whipper-team/whipper/graphs/con
 
 You can find us and talk about the project on:
 
-- IRC: [freenode](https://webchat.freenode.net/?channels=%23whipper), **#whipper** channel
-- Matrix (the room is a bridge to freenode IRC)
-  - Access Matrix  through the [Riot.im web client](https://riot.im/app/#/room/!wxdgcGzudITUpZMCrn:matrix.org)
-  - Join to the room named `#freenode_#whipper:matrix.org`
+- IRC: **`#whipper`** channel on the [Libera.Chat](ircs://irc.libera.chat:6697) network
+  - See [Libera.Chat’s guide on how to connect](https://libera.chat/guides/connect)
+    - Or just use the [Kiwi IRC web client](https://kiwiirc.com/nextclient/irc.libera.chat/?#whipper)
 - [Redacted thread (official)](https://redacted.ch/forums.php?action=viewthread&threadid=150)
 
 Other relevant links: