Add fix as a backport in changelog

Author: cderv

news/changelog-1.9.md

@@ -4,6 +4,7 @@ All changes included in 1.9:
 
 - ([#13396](https://github.com/quarto-dev/quarto-cli/issues/13396)): Fix `quarto publish connect` regression.
 - ([#13441](https://github.com/quarto-dev/quarto-cli/pull/13441)): Catch `undefined` exceptions in Pandoc failure to avoid spurious error message.
+- ([#13046](https://github.com/quarto-dev/quarto-cli/issues/13046)): Use new url for multiplex socket.io server <https://multiplex.up.railway.app/> as default for `format: revealjs` and `revealjs.multiplex: true`.
 
 ## Dependencies