CI: use macos-15 instead of the deprecated macos-13

Author: DanielT

.github/workflows/CI.yml

@@ -117,7 +117,7 @@ jobs:
     strategy:
       matrix:
         platform:
-          - runner: macos-13
+          - runner: macos-15-intel
             target: x86_64
           - runner: macos-14
             target: aarch64