Why this change is needed:
Prepares non-breaking patch releases for google_cloud_firestore and
firebase_admin_sdk by dropping WIP staging suffixes and updating version
constants after landing stability fixes and custom headers support.

Summary of changes:
* Bump google_cloud_firestore version to 0.5.3 in pubspec and changelog.
* Remove -wip suffix from firebase_admin_sdk to stage release 0.5.4.
* Add recent Storage.delete() idempotency bugfix item to changelog.
* Regenerate version.g.dart constant to match 0.5.4.