chore: [SDK-5280] align the example build doc with the iOS 15.1 minimum

[nan-li]

Description

One Line Summary

Align the example build guide with the iOS 15.1 minimum the podspec already declares. SDK-5280.

Details

Motivation

The native iOS SDK is raising its minimum to 15 (OneSignal/OneSignal-iOS-SDK#1749). react-native-onesignal.podspec already declares 15.1, React Native's own floor, so no code change is needed here. The example build guide still told readers to create the notification service extension at 13.0, below the demo project it describes.

Scope

• examples/build-ios.md lists 15.1 for the notification service extension target, matching the demo project.
• No source, podspec or example project changes.

The OneSignal-XCFramework version pin is unchanged. The release that ships the native bump pins it.

Testing

Unit testing

No test changes.

Manual testing

None needed. One line of documentation.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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