chore: [SDK-5280] raise minimum iOS version to 15

[nan-li]

Description

One Line Summary

Raise the minimum iOS version from 12 to 15. SDK-5280.

Details

Motivation

Xcode 27 rejects deployment targets below iOS 15, and the next OneSignal iOS SDK release raises its minimum to 15 (OneSignal/OneSignal-iOS-SDK#1749). A wrapper that declares a lower minimum than the native SDK it depends on cannot be integrated, so it moves with it.

Scope

• The iOS build post-processor stamps the notification service extension with a 15.0 deployment target (was 12.0).
• README and the sample README ask for an iOS 15 or newer device.
• The demo-no-location run script writes a 15.0 Podfile platform (was 13.0).
• The native iOS dependency version is unchanged. The release that ships this pins the iOS SDK release that carries the same bump.

Apps that target iOS 12 to 14 can no longer use this SDK, so the release needs a minor version bump and a release note.

Testing

Unit testing

No test changes.

Manual testing

Not run. The change is one build setting value, one script constant and two README lines.

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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