[3.0] CI: build macOS arm64 with a more recent toolchain

We don't need to use an old toolchain for arm64. Even the latest XCode 26 ¹ still supports making builds compatible with the original MacOS with M1 which was released with macOS Big Sur (macOS 11).

Targeting macOS 10.7 doesn't cause any build issue, even nightlies ².

This will allow the machines tagged monterey for other things. For VLC 4 we may want a newer SDK than the one from XCode 15 (for VLC 3 arm64 as well) monterey is used for macos-x86_64 and ios-armv7 in VLC 4.

cc @ePirat @dfuhrmann

1. https://developer.apple.com/support/xcode/ ↩

2. https://code.videolan.org/robUx4/vlc/-/jobs/2306561 ↩