Update CI Mac builders to use macos-13

This updates `macos.yml` to use `macos-13` instead of
`macos-12`.

macos-12 is deprecated and results in longer queue times
during peak hours. In practice, also failed builds.

For details, see actions/runner-images#10721

.github/workflows/macos.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: macos-12
+    runs-on: macos-13
 
     steps:
     - uses: actions/checkout@v4