* Update to Xcode 15
## Summary
We want to make Texture build with recent tools like Xcode 15. Part of this is bumping the minimum supported OS to iOS 14.
I only removed the dead code (i.e. code gated on versions older than iOS 14). There are still some warnings about outdated APIs in use, but they are non-trivial changes to fix.
## Test plan
Run all the examples. Run all the unit tests and make them pass.
* Use Xcode 15.3 so we have 17.4 simulators
* Fixing an asset catalog compiler error when building against iPhone SE
* Fix a couple of file extensions for assets
A couple of PNGs were marked as JPGs
Andy Finnell
Scott Goodson
Vitaly Baev