Bump to Swift 5.9 (by April 2024)

Apple [will require](https://developer.apple.com/ios/submit/#:~:text=Please%20note%20that,iOS%2017%20SDK.) Xcode 15/Swift 5.9 to deploy apps to the App Store by April 2024. So at some point then I'll bump min Swift version to 5.9. This is just an issue to remind me of it.

Branch: https://github.com/davdroman/swiftui-navigation-transitions/tree/swift-5.9