ci: Use macos-15-intel for x86 macOS tests (#2620)

* As the macos-13 runner images are deprecated use the macos-15-intel runner
  images to test x86 macOS support.
   - c.f. https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/
* This puts an EOL for x86 macOS support in GitHub Actions of Fall 2027.

Author: matthewfeickert

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
           - os: macos-latest
             python-version: '3.12'
           # Intel runner
-          - os: macos-13
+          - os: macos-15-intel
             python-version: '3.12'
 
     steps: