In the Run menu, I can only choose iOS 11 devices (iPhone 5s and newer). How can I build my app for iPhone 5?
You're building your app using the iOS11 SDK. This is not the same thing as the minimum iOS version your app requires.
Simply set your Deployment Target in Info.plist to a version of iOS that the iPhone 5 supports and your app will run on the iPhone 5. However, you won't be able to run your app in a simulator of the iPhone 5 because you're not building it with the SDK for iOS 10.x (the most recent version supported by iPhone 5).
You should still be able to test your app on a physical iPhone 5, though.
To add to this, if you want to test your app on a simulator running iOS 10 you can install simulators with older iOS versions in XCode. Just go to XCode -> Preferences -> Components and download them from there.
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