Update supported platforms docs for iOS with correct SDK information

Our docs currently say that we support the following xcodes for buildin an iOS app.

Xcode 13 (iOS 15 SDK), Xcode 14 (iOS 16 SDK)

https://doc.qt.io/qt-6/ios.html
https://doc.qt.io/qt-6.2/ios.html

Trying to build a sample project
https://github.com/qt/qtbase/blob/dev/tests/manual/ios_assets/ios_assets.pro

with Xcode 13.4.1 + iOS 15.5 SDK on macOS Ventura 13.3 (22E252)
and uploading it to the App store via the Xcode archive "validate" and "distribute" buttons show the error in the first attachment (the one that doesn't say much)

If i try to use the advice at https://stackoverflow.com/questions/71644865/app-store-connect-operation-error-asset-validation-failed

and use the Transporter app https://apps.apple.com/tr/app/transporter/id1450874784?mt=12

Upon validation i get the error in the second attachment.

Given hat the App store says newly submitted apps need to be built with iOS 16.1 + Xcode 14.1, I suggest we update our docs to mention that.