Merging the 6.28.1 release into master (#6049)

* 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

* Add #include for config.h to get the value for HAVE_LIBDISPATCH (#6041) (#6042)

Co-authored-by: Denver Coneybeare <dconeybe@google.com>

* M75.1 version bumps (#6043)

* M75.1: Firestore patch release

* Firestore podspec version bump

* Bump Core version

Co-authored-by: Gil <mcg@google.com>
Co-authored-by: Denver Coneybeare <dconeybe@google.com>

FirebaseCore.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'FirebaseCore'
-  s.version          = '6.9.0'
+  s.version          = '6.9.1'
   s.summary          = 'Firebase Core'
 
   s.description      = <<-DESC