Update CI workflow to use macOS 15

Author: gregcaporaso

template/{{ package_name }}/.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ '{{' }} matrix.os {{ '}}' }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-13]
+        os: [ubuntu-latest, macos-15-intel]
 
     steps:
     - uses: actions/checkout@v2