Update .ci.yaml in flutter/flutter to use iOS 18 only

## Prerequisites

- [x] https://github.com/flutter/flutter/issues/148974

---

https://github.com/flutter/flutter/blob/master/.ci.yaml

```diff
- device_os: iOS-17|iOS-18
+ device_os: iOS-18

- ["device_os=iOS-17","os=Mac-14", "cpu=x86"]
+ ["device_os=iOS-18","os=Mac-14", "cpu=x86"]
```