Xcode validates project build settings with each new Xcode release and
issues warnings to apply recommendations. We ignore these. This patch
sets the validated Xcode version to the maximum so that Xcode will
not longer issue these warnings.
- Remove visionOS from multi-platform builds because it
requires Xcode 15+ and will abort a multi-platform build.
- Define TARGET_OS_XR for older SDK's.
- A number of SDK deprecation warnings remain when building for visionOS.
These cannot be removed without significant refactoring.
- Build visionOS dependencies for Release build by default.
- Fix local variable initialization warning (unrelated).
