CD: pin GNU Linux builds to ubuntu-22.04 for Debian 12 compat (#2135)

- Fix .deb packages being uninstallable on Debian 12 due to glibc 2.39 dependency
  (caused by building on ubuntu-latest/24.04 instead of ubuntu-20.04).
- Pin GNU Linux builds to ubuntu-22.04 (glibc 2.35).
- Add post-release verification of asset completeness and Debian 12 compatibility. Create releases as drafts so release notes can be written before the notification email goes out.