Flutter: update iOS minimum deployment target to 15.0 and Xcode to 26

[cadivus]

Follow-up to aws-amplify/amplify-flutter#7239, which raised the iOS deployment target of the Amplify Flutter plugins from 13.0 to 15.0.

• iOS minimum deployment target: 13.0 -> 15.0
• Minimum Xcode (iOS and macOS): 15.0 -> 26.0. This does not come from feat: Add iOS 27 support amplify-flutter#7239, which leaves the Xcode floor untouched. Apple has required Xcode 26 and the iOS 26 SDK for App Store uploads since April 28, 2026 (https://developer.apple.com/news/?id=ueeok6yw), and amplify-flutter CI builds on macos-26 runners, so the previous 15.0 was stale.
• macOS deployment target unchanged (10.15)
• Fixed the macOS Podfile code block title, which said ios/Podfile

Draft because the amplify-flutter change is not released yet.

The screenshots on this page still show the old versions.