Add backport to changelog for #13046

Author: cderv

news/changelog-1.8.md

@@ -2,6 +2,8 @@
 
 ## In this release
 
+- ([#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`.
+
 ## In previous releases
 
 - ([#13369](https://github.com/quarto-dev/quarto-cli/issues/13369)): Fix failure in theme compilation when `brand.color.primary` is specified for light or dark but not both.