docs: update Ubuntu info for electron sandboxing

starting in Ubuntu 23.10, and officially in 24.04, userns is disable in
Ubuntu. Add information about userns working prior to 24.04 and linking
to the Ubuntu release notes explaining userns and workarounds

source/user-guide/troubleshooting/electron-sandboxing.rst

@@ -3,7 +3,7 @@
 I have issues with Electron-based AppImages and their sandboxing
 ================================================================
 
-AppImages based on `Electron <https://www.electron.build/>`__ require the kernel to be configured in a certain way to allow for its sandboxing to work as intended (specifically, the kernel needs to be allowed to provide "unprivileged namespaces"). Many distributions come with this configured out of the box (like `Ubuntu <https://ubuntu.com>`__ for instance), but some do not (for example `Debian <https://debian.org>`__).
+AppImages based on `Electron <https://www.electron.build/>`__ require the kernel to be configured in a certain way to allow for its sandboxing to work as intended (specifically, the kernel needs to be allowed to provide "unprivileged namespaces"). Many distributions come with this configured out of the box (like `Ubuntu <https://ubuntu.com>`__ prior to `24.04 <https://discourse.ubuntu.com/t/ubuntu-24-04-lts-noble-numbat-release-notes/39890#unprivileged-user-namespace-restrictions-15>`__), but some do not (for example `Debian <https://debian.org>`__).
 
 .. warning::