Resolve merge conflicts from release-6.0.0 branch (#2958)

* Update versions for Release 6.0.0

* Update FirebaseSDKs.textproto with new fields. (#2818) (#2834)

* Update FirebaseSDKs.textproto with new fields.

* Update message about strip_32bits flag.

* GULAppDelegateSwizzler - proxy APNS methods separately (#2835) (#2858)

* Cherry-picks for M47 RC5 (#2882)

* fix race condition checkin is deleted before writing during app start (#2860)

* GoogleUtilities min ios version from 6 to 8 (#2876)

* Put secret and oauth credentials into the private headers for now. (#2916) (#2919)

* Add missing Messaging weak_framework dep (#2921) (#2930)

FirebaseDatabase.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'FirebaseDatabase'
-  s.version          = '5.1.1'
+  s.version          = '6.0.0'
   s.summary          = 'Firebase Open Source Libraries for iOS (plus community support for macOS and tvOS)'
 
   s.description      = <<-DESC