Although I had accepted the answer from the App Store Connect Engineer (as it does get around the error in Transporter 1.3), it presented a new problem after uploading the app. Without the LSMinimumSystemVersion key in the Info.plist, I get an email from App Store Connect with the following issue:
ITMS-90899: Macs with Apple silicon support issue - The app isn‘t compatible with the provided minimum macOS version of 11.5. It can run on macOS 12.0 or later. Please specify an LSMinimumSystemVersion value of 12.0 or later in a new build, or select a compatible version in App Store Connect. For details, visit: https://developer.apple.com/help/app-store-connect/manage-your-apps-availability/manage-availability-of-iphone-and-ipad-apps-on-macs-with-apple-silicon.
The app minimum deployment is iOS 14.7.
I tried setting the macOS minimum version for the app via the instructions (on App Store Connect - Pricing and Availability page) but I still get the issue email. This happens if I set it to Automatic (11.5) or 12.0. I get the same response regardless of the selected version.
If I add the key back in, Transporter 1.3 has the same issue.
If I use the key without LS as suggested by the engineer, then I get the email (I assume that this is just not recognized as it is not documented).
If I restore the key as documented (LSMinimumSystemVersion) and I use the prior Transporter (1.2.5), all works as it did before (no error and no issue email).
I think this is just an error in Transporter 1.3 when using this key as documented. However, the fact that without the key, the setting on App Store Connect is not honored is also of concern. It may have something to do with the app iOS 14.7 minimum deployment...but that had not been an issue when I was able to specify the key in the Info.plist with the prior Transporter app version.