Switch to Mac 15 in CI

Author: hedtke

.github/workflows/main.yml

@@ -77,7 +77,7 @@ jobs:
       - name: Run tests
         run: ./test/tests
   test_release_mac:
-    runs-on: macos-13
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@v3
       - uses: turtlebrowser/get-conan@main