Switch to macos-latest (currently macos-12) for OSX github runner build

Author: mmp

.github/workflows/ci-mac.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ macos-15 ]
+        os: [ macos-latest ]
 
     name: Build ${{ matrix.os }}