Move the minimum iOS version from 13.0 to 15.0

[jkingok]

Change the minimum iOS version for Xcode

On newer versions of Xcode, including Xcode 27, the current version of 13.0 is now rejected and CLI-based automated builds fail, either the project needing to be manually updated or briefcase open iOS run to change the setting.

Also, on submission to App Store Connect/TestFlight Apple also complains and warns they won't support iOS < 15 shortly.

To fix both move the default forwards to iOS 15.

This reduces friction and removes some boilerplate (e.g. I was overriding this in CI to this point using fastlane plugins).

Notice that the GUI of Xcode 27 does not actually allow you to choose anything older than iOS 17, so we could choose that, but I expect we have a project policy to be conservative and tolerant of older versions of Xcode for as long as possible.

Tested using template argument in a Toga testbed with Xcode 27.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

If approved a change note will need to follow in the briefcase repo.

[freakboy3742]

Thanks for the update - agreed that this change seems inevitable. There's a related conversation that is needed with the CPython core team about lifting the minimum supported iOS version, but we can make this change without that conversation being resolved.

There's a corresponding change needed in Briefcase itself - we need to add a changenote so that users are warned that newly generated projects will default to an iOS 15 minimum. We should probably take this opportunity to document that minimum version in the iOS Xcode platform documentation as well.

Approving this PR, but not merging until that changenote is ready to merge. Let me know if you're not able to write up that PR and docs change and I'll add it to my todo list.