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

FirebaseCore.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'FirebaseCore'
-  s.version          = '6.8.1'
+  s.version          = '6.9.0'
   s.summary          = 'Firebase Core'
 
   s.description      = <<-DESC
@@ -45,7 +45,7 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
   s.pod_target_xcconfig = {
     'GCC_C_LANGUAGE_STANDARD' => 'c99',
     'GCC_PREPROCESSOR_DEFINITIONS' =>
-      'FIRCore_VERSION=' + s.version.to_s + ' Firebase_VERSION=6.27.1',
+      'FIRCore_VERSION=' + s.version.to_s + ' Firebase_VERSION=6.28.0',
     'HEADER_SEARCH_PATHS' => '"${PODS_TARGET_SRCROOT}"',
     'OTHER_CFLAGS' => '-fno-autolink'
   }