Actions: Switch to macos-15 (#394)

Co-authored-by: Johannes Lorenz <jlsf2013@sourceforge.net>

Author: JohannesLorenz

.github/workflows/build.yml

@@ -79,7 +79,7 @@ jobs:
         run: make install
   macos:
     name: macos
-    runs-on: macos-13
+    runs-on: macos-15
     steps:
       - name: Install python3
         run: brew install --overwrite python@3.12