It seems that the latest update of AdMob 11.2 released on 14 March 2024 include a work around to address this issue. With this package updated, the new build made using Xcode 15.3, is accepted by App Store Connect as expected.

I experienced the same issue with Google AdMob (v11) after upgrading to Xcode 15.3. Downgrading to Xcode 15.2, clearing the Swift Package Cache, and rebuilding worked. I am not sure why that is though. :-)