maint(ios): upgrade to Xcode 26 required before April 2026

[mcdurdin]

Message in 19.0.137 alpha release build:

ITMS-90725: SDK version issue - This app was built with the iOS 18.2 SDK. Starting April 2026, all iOS and iPadOS apps must be built with the iOS 26 SDK or later, included in Xcode 26 or later, in order to be uploaded to App Store Connect or submitted for distribution.

[sgschantz]

The build agents are running on macOS 15.3, so we will also have to upgrade them to at least macOS 15.6 to run Xcode 26.

Versions supported for each release of Xcode are found here.

It would probably be best to upgrade to macOS 26 (Tahoe) everywhere just to stay in sync.