3616 [ios_platform_images] Bump version for stable NNBD (cla: yes, nnbd, p: ios_platform_images, platform-ios, waiting for tree to go green)

3618 [quick_action] null safety stable release (cla: yes, p: quick_actions, waiting for tree to go green)

3621 [quick_action] fix delegate methods not called on iOS (cla: yes, p: quick_actions, platform-ios, waiting for tree to go green)

3623 [google_maps_flutter] Bump app-facing version for NNBD stable (cla: yes, nnbd, p: google_maps_flutter, waiting for tree to go green)

3628 [google_sign_in_web] Migrate to null-safety (cla: yes, nnbd, p: google_sign_in, platform-web, waiting for tree to go green)