docs: Update IRC links to Libera.Chat

#mgmtconfig has moved to Libera.Chat as the primary channel for IRC
communications. Update the documentation to reflect this.
Libera.Chat doesn't provide a first-party web portal but does recommend
a few in the linked documentation on the website. As there is no
suitable replacement for webchat.freenode.net, link to the "Choosing an
IRC client" page instead.

Signed-off-by: Joe Groocock <me@frebib.net>

README.md

@@ -6,7 +6,7 @@
 [![Build Status](https://img.shields.io/travis/purpleidea/mgmt/master.svg?style=flat-square)](http://travis-ci.org/purpleidea/mgmt)
 [![Build Status](https://github.com/purpleidea/mgmt/workflows/.github/workflows/test.yaml/badge.svg)](https://github.com/purpleidea/mgmt/actions/)
 [![GoDoc](https://img.shields.io/badge/godoc-reference-5272B4.svg?style=flat-square)](https://godoc.org/github.com/purpleidea/mgmt)
-[![IRC](https://img.shields.io/badge/irc-%23mgmtconfig-orange.svg?style=flat-square)](https://webchat.freenode.net/?channels=#mgmtconfig)
+[![IRC](https://img.shields.io/badge/irc-%23mgmtconfig-orange.svg?style=flat-square)](ircs://irc.libera.chat:6697/#mgmtconfig)
 [![Patreon](https://img.shields.io/badge/patreon-donate-yellow.svg?style=flat-square)](https://www.patreon.com/purpleidea)
 [![Liberapay](https://img.shields.io/badge/liberapay-donate-yellow.svg?style=flat-square)](https://liberapay.com/purpleidea/donate)
 
@@ -66,7 +66,7 @@ Come join us in the `mgmt` community!
 
 | Medium | Link |
 |---|---|
-| IRC | [#mgmtconfig](https://webchat.freenode.net/?channels=#mgmtconfig) on Freenode |
+| IRC | [#mgmtconfig](ircs://irc.libera.chat:6697/#mgmtconfig) on Libera.Chat |
 | Twitter | [@mgmtconfig](https://twitter.com/mgmtconfig) & [#mgmtconfig](https://twitter.com/hashtag/mgmtconfig) |
 | Mailing list | [mgmtconfig-list@redhat.com](https://www.redhat.com/mailman/listinfo/mgmtconfig-list) |
 | Patreon | [purpleidea](https://www.patreon.com/purpleidea) on Patreon |