Looks like CocoaPods 1.12.1 is incompatible with < Xcode 14 due to https://github.com/CocoaPods/CocoaPods/pull/11828 (see https://github.com/flutter/flutter/issues/123890 for context).

Bump the minimum Xcode version allowed by tooling to 14, released September 2022.

```
[!] Xcode - develop for iOS and macOS (Xcode 13.4)
    ✗ Flutter requires Xcode 14 or higher.
      Download the latest version or update via the Mac App Store.
```

Fixes https://github.com/flutter/flutter/issues/125286.
Previous bump at #97746.