github_runner_matrix: enable usage of macos-15 runners

[carlocab]

• Have you followed the guidelines in our Contributing document?
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https://github.blog/changelog/2024-09-25-actions-new-images-and-ubuntu-latest-changes/
https://github.com/actions/runner-images/blob/ae99c16b0cf27c0cca7ef0b08e7bcf13c33f6cfa/README.md#available-images