Fix CD: replace defunct ubuntu-20.04 runners (#2129)

* Fix CD: replace discontinued ubuntu-20.04 runners; update Actions

ubuntu-20.04 runners were removed from GitHub Actions, causing 4 of 7
CD build targets to time out waiting for a runner (issue #2127).

- Replace ubuntu-20.04 with ubuntu-latest in cd.yml
- Update actions/checkout v3 → v4 (Node.js 20 deprecation)
- Update softprops/action-gh-release v1 → v2