Update Xcode to 16.4

[targos]

Now required by V8, which uses simdutf::atomic_binary_to_base64, which requires std::atomic_ref support, which is implemented in LLVM 19, which is included in Xcode 16.3.

This will also require a macOS bump to at least 15.2.

Refs: v8/v8@e3cddbe, llvm/llvm-project#76647, https://en.wikipedia.org/wiki/Xcode, https://chromium-review.googlesource.com/c/chromium/src/+/6431799
Refs: #56824, #56831

[targos]

Chromium now uses Xcode 16.4 (which requires macOS 15.3 at least): https://chromium-review.googlesource.com/c/chromium/src/+/6674373