CI: upgrade workflows to use macos-15(*) rather than macos-13

[marcelosalloum]

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What

Upgrade workflows to use macos-15-* rather than macos-13

Why

1. It's about to be deprecated: announcement 🔗
2. It's already making the CI fail: example run 🔗

Notes on Runner Compatibility

After extensive testing across macOS runner types:

Runner	Result	Notes
✅ macos-15-large	✅ Passes	Stable with full setup.
✅ macos-14	✅ Passes	Works consistently.
🟠 macos-15-intel	⚠️ Intermittent	Suitable for lighter setups but fails under heavier load — -large is usually more stable.
🟠 macos-15	⚠️ It depends	Does not support nested virtualization (required by Lima/QEMU), so cannot be used in some setups.