firebase-ios-sdk

Merging the 6.28.0 release into master (#6040)

* Update versions for Release 6.28.0

* Release 6.28.0 - GDT cherry pick of #6010 and #6032 (#6035)

* [GDTCORUploadCoordinator startTimer]: don't restart timer if already started (#6032)

* [GDTCORUploadCoordinator startTimer]: don't restart timer if already started

* style

* `GDTCORUploadCoordinator.timer` - nullable

* GDTCORUploadCoordinator test utils thread safety.

* GDTCORFlatFileStorage: keep not expired events when expired batch removed (#6010)

* GDTCORFlatFileStorage: expiration clean up tests

* GDTCORFlatFileStorage: keep not expired events when expired batch removed

* Cleanup

* Cleanup

* Comments

* Comments

* Remove additional delays in tests

* style

* Changelog

* Changelog: remove 7.0.1