[macOS] Update default Xcode to 16.4 on macOS-15 (#12871)

Author: erik-bershel

images/macos/toolsets/toolset-15.json

@@ -1,6 +1,6 @@
 {
     "xcode": {
-        "default": "16",
+        "default": "16.4",
         "x64": {
             "versions": [
                 {