ci: remove macos-10.15 job, it has been deprecated

Author: danmar

.github/workflows/CI-unixish.yml

@@ -9,7 +9,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, ubuntu-22.04, macos-10.15, macos-11, macos-12]
+        os: [ubuntu-18.04, ubuntu-20.04, ubuntu-22.04, macos-11, macos-12]
       fail-fast: false # Prefer quick result
 
     runs-on: ${{ matrix.os }}