Update .ci.yaml in flutter/flutter to use iOS 18 or 26

## Prerequisites

- [x] https://github.com/flutter/flutter/issues/172858
- [ ] Ensure no open issues in https://github.com/flutter/flutter/issues/186088

---

https://github.com/flutter/flutter/blob/master/.ci.yaml

```diff
- device_os: iOS-18
+ device_os: iOS-18|iOS-26

- ["device_os=iOS-18","os=Mac-15.7", "cpu=x86"]
+ ["device_os=iOS-18|iOS-26","os=Mac-15.7", "cpu=x86"]
```