This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/wiki/Flutter-Cherrypick-Process#automatically-creates-a-cherry-pick-request)
Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

Part of https://github.com/flutter/flutter/issues/144582#issuecomment-2040653877

### Changelog Description:
Increase the `flutter doctor` recommended Xcode version to Xcode 15.

### Impact Description:

Update the `flutter doctor` validator to suggest Xcode 15 if installed version is lower.

This is to encourage upgrades, as Apple will soon be enforcing this during iOS App Store submissions.
> Please note that as of April 2024 all iOS and iPadOS apps submitted to the App Store must be built with a minimum of Xcode 15 and the iOS 17 SDK.

https://developer.apple.com/ios/submit/

It's also to encourage updates before Swift Package Manager requires this version in https://github.com/flutter/flutter/pull/146256.

### Workaround:
N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
Install and `xcode-select` Xcode 14.x or lower.  Run `flutter doctor`.  See 
```
[!] Xcode - develop for iOS and macOS (Xcode 15.3)
    ✗ Flutter requires Xcode 14 or higher.
      Download the latest version or update via the Mac App Store.
```