4352 [in_app_purchase] Bump minimum Flutter version and iOS deployment target (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-ios)

4363 [in_app_purchase] Fix in_app_purchase_android/README.md (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-android)

4364 [in_app_purchase] Ensure the introductoryPriceMicros field is populated correctly. (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-android)

4367 Require authors file (cla: yes, p: camera, p: in_app_purchase, p: quick_actions, platform-ios, platform-android, platform-web)