Update iOS deployment target to 15.0

[htenlik]

What did you change and why?

Updated the iOS deployment target from 12.0 to 15.0.

This is required to build TrueTime with Xcode 27 / the iOS 27 SDK, which no longer supports an iOS 12 deployment target.

This change is part of the iOS 27 compatibility work in LoopKit/Loop#2492.

Potential risks introduced?

Low risk.

This only raises the minimum supported iOS version for this project configuration and does not change runtime behavior or library logic.

What tests were performed (include steps)?

Tested as part of a Loop workspace build using:

• macOS 26.5.2
• Xcode 27 beta 5
• iOS 27
• Physical iPhone 16 Plus

Steps:

1. Updated the deployment target to iOS 15.0.
2. Built the Loop workspace with Xcode 27 beta 5.
3. Installed and launched the resulting Loop build on an iOS 27 device.
4. Exercised normal application navigation and usage.

The build completed successfully and no launch-time crashes or regressions were observed during testing.

Checklist

• Unit/UI tests have been written (if necessary)
• Manually tested