Raise Swift iOS deployment target to 15

[kieran-osgood-shopify]

What changes are you making?

Raise the Swift Checkout Kit iOS deployment floor from iOS 13 to iOS 15 across SwiftPM and CocoaPods metadata, update the Swift API digester target, and refresh Swift docs.

This also removes now-unnecessary iOS < 15 compatibility branches in the Swift SDK/sample code, and aligns the React Native iOS podspec/README minimum because the RN iOS pod depends on the native Swift ShopifyCheckoutKit pod.

How to test

• shadowenv exec --dir <repo_root> -- grep -R "iOS 13\|ios13\|\.iOS(.v13)\|deployment_target = \"13.0\"" Package.swift ShopifyCheckoutKit.podspec platforms/swift platforms/react-native/modules/@shopify/checkout-kit-react-native/RNShopifyCheckoutKit.podspec platforms/react-native/README.md — no matches
• shadowenv exec --dir <repo_root> -- /opt/dev/bin/dev swift test — passed
• shadowenv exec --dir <repo_root> -- /opt/dev/bin/dev swift build — passed
• shadowenv exec --dir <repo_root> -- /opt/dev/bin/dev swift lint — passed
• shadowenv exec --dir <repo_root> -- /opt/dev/bin/dev swift api check — passed; no API baseline updates required
• shadowenv exec --dir <repo_root> -- /opt/dev/bin/dev react-native lint — passed

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Before you merge

Important

• I have added tests to support my implementation
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I have updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

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Releasing a new Swift version?

• I have bumped the version in ShopifyCheckoutKit.podspec
• I have bumped the version in platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
• I have updated platforms/swift/CHANGELOG.md
• I have updated the SwiftPM/CocoaPods version snippets in platforms/swift/README.md (major version only)

Releasing a new Android version?

• I have bumped the versionName in platforms/android/lib/build.gradle
• I have updated platforms/android/CHANGELOG.md
• I have updated the Gradle/Maven version snippets in platforms/android/README.md

Tip

See the Contributing documentation for the full release process per platform.

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