Version 6 is a large under the hood refactor / upgrade that brings the project up to modern angular standards. There have been no functionality changes.
@ionic/storage-angular: This package now requires the @ionic/storage-angular package at version 3 rather than @ionic/storage.
Steps for upgrade
npm rm @ionic/storage
npm install @ionic/storage-angular@latest
Angular 12: This package now uses Angular 12. The minimum supported version for the package is now Angular 9.