Only mention Matrix channel as chat room

We'd like to direct all communications to the Matrix channel going
forward, so let's drop mentions of the IRC channel.

Related: #1566

README.md

@@ -27,15 +27,8 @@ technologies and produce Fedora CoreOS.
 
 - main mailing list: [coreos@lists.fedoraproject.org](https://lists.fedoraproject.org/archives/list/coreos@lists.fedoraproject.org/)
 - status mailing list: [coreos-status@lists.fedoraproject.org](https://lists.fedoraproject.org/archives/list/coreos-status@lists.fedoraproject.org/) (announcements/important messages)
-- Chat rooms:
-  - Matrix: [`#coreos:fedoraproject.org` on Matrix](https://matrix.to/#/#coreos:fedoraproject.org)
-  - IRC: [`#fedora-coreos` on Libera.Chat](https://web.libera.chat/#fedora-coreos) (ircs://irc.libera.chat:6697/#fedora-coreos)
-  - As of 2023-08-06 14UTC, the two rooms are not bridged together anymore thus
-    writing a message to IRC will not reach Matrix users (and vice versa). Note
-    that a lot of CoreOS developers have moved to Matrix thus the IRC channel
-    is likely to get less attention and we recommend joining via Matrix. See
-    [Matrix to libera.chat (IRC) bridge unavailable](https://communityblog.fedoraproject.org/matrix-to-libera-chat-irc-bridge-unavailable/).
-    The meetings still happen on IRC for now.
+- Chat room: [`#coreos:fedoraproject.org` on Matrix](https://matrix.to/#/#coreos:fedoraproject.org)
+  - Note that meetings still happen on IRC for now (see below).
 - forum at [https://discussion.fedoraproject.org/tag/coreos](https://discussion.fedoraproject.org/tag/coreos)
 - feature planning and important issue tracking at [github.com/coreos/fedora-coreos-tracker](https://github.com/coreos/fedora-coreos-tracker)
 - website at [https://getfedora.org/coreos/](https://getfedora.org/coreos/)

docs/ci-and-builds.md

@@ -51,7 +51,7 @@ Examples:
 
 ## quay.io/coreos-assembler namespace
 
-A key aspect of Fedora CoreOS as well as RHEL CoreOS is [coreos-assembler](https://github.com/coreos/coreos-assembler).  As of today, we build it in quay.io and deliver it that way in the `quay.io/coreos-assembler` namespace.  The list of administrators for this namespace is managed independently of anything else.  If you think you need administrator access, file a ticket or ask on #fedora-coreos IRC.
+A key aspect of Fedora CoreOS as well as RHEL CoreOS is [coreos-assembler](https://github.com/coreos/coreos-assembler).  As of today, we build it in quay.io and deliver it that way in the `quay.io/coreos-assembler` namespace.  The list of administrators for this namespace is managed independently of anything else.  If you think you need administrator access, file a ticket or ask on `#coreos:fedoraproject.org` on Matrix.
 
 ### The buildroot container: quay.io/coreos-assembler/fcos-buildroot:testing-devel