Add Ubuntu 22.04 support for Linux builds

This is a small workflow-only change that moves the Linux CI and release builds to Ubuntu 22.04. Ubuntu 22.04 is about four years old now, but it is still the baseline in some environments, including Ubuntu AWS WorkSpaces, so building there should help keep the released Linux artifacts usable on those systems without changing any product code.

I used Codex to prepare the workflow update, and I also built and tested the app directly on Ubuntu 22.04, where it worked fine. The full PR checks are passing as well, including the JavaScript test suite and Linux Tauri build.