v7.0.0

GMA SDK major version update 7.0.0 (#1369)

* Fix Native Ads templates

* Updated iOS project, updated inline banner ad unit in example app

* Correctly set new low bounds for Flutter and Dart versions

* Updated minSdk for Android to make it compatible with Flutter versions

* Format fix

* Added more contraints on Native Ad template to avoid unexpected button behaviors

* Made the release a major release (7.0.0)

* Updated Android GMA SDK to 24.9.0

* Modified layout contraints to let CTA button in Native Ad to not be pushed away