Upgrade from deprecated macos-13 to macos-15-intel in CI

[cclauss]

Replace GitHub Actions runner image macos-13 with macos-15-intel as recommended in:

• https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down

The macOS 13 runner image will be retired by December 4th, 2025.

The currently available GitHub Actions macOS runners are:

macOS Version	runner.arch
macos-13	X64
macos-14	ARM64
macos-15	ARM64
macos-15-intel	X64
macos-26	ARM64
macos-latest	ARM64

[github-actions]

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