You're guess is right. You need to update Xcode installation on the Mac which uploads the Testflight builds.
If it's a CI machine or some kind of cloud build service like Bitrise
, you have to update/select a newer Xcode version. If you're submitting Testflight builds from your local Mac, update Xcode locally.
You can check on Wikipedia, which iOS SDK was introduced in which Xcode version:
https://en.wikipedia.org/wiki/Xcode
A quick summary:
Xcode version iOS Base SDK included
8.0 10.0
8.1 10.1
8.2 10.2
8.3 10.3
8.3.3 10.3.1
9.0 11.0
9.1 11.1
9.2 11.2
9.3 11.3
9.4 11.4
10.0 12.0
10.1 12.1
10.2 12.2
10.3 12.4
11.0 13.0
11.1 13.1
11.2 13.2
12.0 14.0
12.1 14.1
12.2 14.2
12.3 14.3
12.4 14.4
12.5 14.5
13.0 15.0
13.1 15.0
13.2 15.2
13.3 15.4
Every Xcode release is packed with a specific version of iOS Base SDK. You can't choose which Base SDK you want to use, only that particular version is available.
For example in Xcode 11.1
you can only use iOS 13.1
SDK. iOS 13.0
or iOS 13.2
SDK is not available.
Note: While you're picking the latest SDK with Xcode updates to access new features, you can still use for example iOS 10.0 Deployment Target
in your project to support older devices.