GoogleMaps 4.1.0 released in December supported iOS 9:
https://github.com/CocoaPods/Specs/blob/63bad7fbc7b87238db00d4003ac0d269d1108110/Specs/a/d/d/GoogleMaps/4.1.0/GoogleMaps.podspec.json#L14

4.2.0 dropped it:
https://github.com/CocoaPods/Specs/blob/master/Specs/a/d/d/GoogleMaps/4.2.0/GoogleMaps.podspec.json

Update the google_maps_flutter.podspec.