GH Actions: bump to macOS 15 (was 14)

Author: generalmimon

.github/workflows/build.yml

@@ -40,7 +40,7 @@ jobs:
       - name: Run tests
         run: .build/run-unittest --valgrind
   macos:
-    runs-on: macos-14
+    runs-on: macos-15
     strategy:
       fail-fast: false
       matrix: