Drop now-unsupported Mac 13 from CI

Author: pimterry

.github/workflows/ci.yml

@@ -108,9 +108,6 @@ jobs:
           - platform: MacOS 14 (ARM64)
             os: "macos-14"
             test-script: "./unix.sh darwin arm64"
-          - platform: MacOS 13
-            os: "macos-13"
-            test-script: "./unix.sh darwin x64"
           - platform: Windows 2025
             os: "windows-2025"
             test-script: ".\\windows.bat"