Switch our macOS builds to use a macOS 13 GitHub Actions runner image.

GitHub deprecated the macOS 12 Actions runner image in October 2024,
and since early December 2024 it is fully unsupported. So let's use
the next available version then.

.github/workflows/macos.yml

@@ -8,14 +8,14 @@ env:
   REPO_DIR : ${{github.workspace}}
   BUILD_DIR: ${{github.workspace}}/bin/builddir
   BOOST_VERSION: "1.83.0"
-  BOOST_PLATFORM_VERSION: "12"
+  BOOST_PLATFORM_VERSION: "13"
 
 permissions:
   contents: read
 
 jobs:
   build:
-    runs-on: macos-12
+    runs-on: macos-13
     permissions:
       contents: read