Remove Mac 12 (now deprecated) from CI and add Mac 15 (beta)

Author: pimterry

.github/workflows/ci.yml

@@ -96,15 +96,15 @@ jobs:
           - platform: Ubuntu 20.04
             os: "ubuntu-20.04"
             test-script: "./unix.sh linux"
+          - platform: MacOS 15 (M1)
+            os: "macos-15"
+            test-script: "./unix.sh darwin"
           - platform: MacOS 14 (M1)
             os: "macos-14"
             test-script: "./unix.sh darwin"
           - platform: MacOS 13
             os: "macos-13"
             test-script: "./unix.sh darwin"
-          - platform: MacOS 12
-            os: "macos-12"
-            test-script: "./unix.sh darwin"
           - platform: Windows 2022
             os: "windows-2022"
             test-script: ".\\windows.bat"