Drop MacOS 10.15 from CI testing now that it's really gone

Latest build no longer runs - just spins waiting for a worker
indefinitely. EOL was September 2022. Time to move on.

Author: pimterry

.github/workflows/ci.yml

@@ -100,9 +100,6 @@ jobs:
           - platform: MacOS 11
             os: "macos-11"
             test-script: "./unix.sh darwin"
-          - platform: MacOS 10.15
-            os: "macos-10.15" # Deprecated - nice-to-test now, but remove this entirely once GitHub stops supporting it
-            test-script: "./unix.sh darwin"
           - platform: Windows 2022
             os: "windows-2022"
             test-script: ".\\windows.bat"