Remove no-longer-available Ubuntu 20.04 from distributable testing

GitHub dropped this recently, so we can't automatically test it, and
it's also obviously quite outdated now anyway.

Author: pimterry

.github/workflows/ci.yml

@@ -96,9 +96,6 @@ jobs:
           - platform: Ubuntu 22.04 x64
             os: "ubuntu-22.04"
             test-script: "./unix.sh linux x64"
-          - platform: Ubuntu 20.04 x64
-            os: "ubuntu-20.04"
-            test-script: "./unix.sh linux x64"
           - platform: MacOS 15 (ARM64)
             os: "macos-15"
             test-script: "./unix.sh darwin arm64"