すでにiPadとiPhoneをサポートしていたアプリについて、iPadのサポートを外して、App Store Connectにアップロードしようとしたところ以下のエラーが出ました。

*** Error: This bundle does not support one or more of the devices supported by the previous app version. Your app update must continue to support all devices previously supported. You declare supported devices in Xcode with the Targeted Device Family build setting. Refer to QA1623 for additional information: https://developer.apple.com/library/ios/#qa/qa1623/_index.html App Store operation failed. (1091)
 {
    NSLocalizedDescription = "This bundle does not support one or more of the devices supported by the previous app version. Your app update must continue to support all devices previously supported. You declare supported devices in Xcode with the Targeted Device Family build setting. Refer to QA1623 for additional information: https://developer.apple.com/library/ios/#qa/qa1623/_index.html";
    NSLocalizedFailureReason = "App Store operation failed.";
    NSLocalizedRecoverySuggestion = "This bundle does not support one or more of the devices supported by the previous app version. Your app update must continue to support all devices previously supported. You declare supported devices in Xcode with the Targeted Device Family build setting. Refer to QA1623 for additional information: https://developer.apple.com/library/ios/#qa/qa1623/_index.html";
}

一度サポートしたデバイスのサポートを外すことはできないようです。

ドキュメント https://developer.apple.com/library/archive/qa/qa1623/_index.html を見てみると、以下のような対策が書かれています。

全く新しいアプリとして開発する（新しいアプリはiPhone専用とし、iPadは外す、ということでしょう）
要求iOSバージョンをあげることで、古い端末へのサポートを外す（しかしiPadシリーズは新しいiOSバージョンにも対応していくので、今回はこれでは対策にならなそう）
諦めて、以前と同様の端末をサポートし続ける

Q: Why am I getting an error about supporting previous devices when I upload an update of my app?
A: iTunes Connect does not allow uploading an updated version of an app when the update runs on fewer devices than the version of the app currently in the App Store. This is by design.
An update to an app must work for every customer who has already purchased the app, and is running a current version of iOS.
Developers who wish to issue updates, but remove device support, have three choices:
Fix their app so that it can work on the devices they originally set out to support. See Supporting All Devices for details.
Target a newer version of iOS that requires a newer device. See Targeting the Latest iOS for details.
Remove their app from the store, and upload the new app with a different bundle ID. See Replacing Your App in the Store for details.