[image_picker_ios] Require Flutter 3.47 / iOS 15 and drop deprecated kUTType code paths (#13224)

With Xcode 27 the `xcode analyze` step of `Mac_arm64 ios_platform_tests` fails on image_picker_ios:

```
FLTImagePickerImageUtil.m:132:37: error: 'kUTTypeGIF' is deprecated: first deprecated in iOS 15.0 - Use UTTypeGIF or UTType.gif (swift) instead. [-Werror,-Wdeprecated-declarations]
  132 |     gifTypeIdentifier = (NSString *)kUTTypeGIF;
```

Staging run on Xcode 27 (27A266a): https://ci.chromium.org/b/8668491298933206513

`kUTTypeGIF` is only referenced in the `else` branch of an `if (@available(iOS 14.0, *))` check (added in #10848), so it never runs on iOS 14+. The compiler still has to compile that branch though, and Xcode 27 only supports deployment targets of iOS 15 and up, so the plugin is now always built with `-target …-ios15.0` and the reference trips `-Wdeprecated-declarations`, which `xcode analyze` treats as an error. The build stops at the first failing file; `FLTImagePickerPhotoAssetUtil.m` has the same pattern, and `FLTImagePickerPlugin.m` uses `kUTTypeImage` / `kUTTypeMovie` unconditionally.

Flutter 3.47 dropped iOS 13 and 14, so rather than keep those code paths alive this raises the package minimum to Flutter 3.47 / Dart 3.13 and iOS 15 (podspec and `Package.swift`), and deletes the iOS 13 branches: `UTTypeGIF`, `UTTypeImage` and `UTTypeMovie` are used directly. Clients on older Flutter stay on the previous version through `pub`, so this is a patch bump, same as #10231 did for iOS 13. The remaining `@available(iOS 13/14, *)` checks in `FLTImagePickerPlugin.m` are now dead too; I left them alone since that file is being rewritten in Swift in #13235 and they can go there.

Also updates the app-facing `image_picker` README's support table to iOS 15 (with a `## NEXT` entry, no version bump), the same way #10470 did for iOS 13.

Verified on Xcode 27 (with the earlier, fallback-preserving version of this change): `xcode analyze`, `xcode analyze deprecation` and the native tests pass for image_picker_ios — https://ci.chromium.org/b/8668086845504323249 (shard 3 of a staging replay; the remaining `native test` failure in that build is in_app_purchase_storekit, https://github.com/flutter/flutter/issues/194285).

Part of https://github.com/flutter/flutter/issues/194019

## Pre-Review Checklist

Test exemption: native-only change with no behaviour change; the existing GIF tests in `RunnerTests/ImageUtilTests.swift` and `RunnerTests/PhotoAssetUtilTests.swift` already exercise both call sites.

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