ci: Restore packaging with macOS SDK from Xcode

We now build with Qt 6.9.3, which supports the macOS 26 SDK.
Revert commit baa8cab401 (ci: Revert packaging to macOS 15.5 SDK to
restore cmake-gui rendering, 2025-10-25, v4.2.0-rc2~19^2) and drop
supporting scripts.

Issue: #27325
5 files changed