Use at least Protobuf 3.9.2 (#3916)

Author: paulb777

FirebaseABTesting.podspec

@@ -42,7 +42,7 @@ Firebase Cloud Messaging and Firebase Remote Config in your app.
   }
   s.dependency 'FirebaseAnalyticsInterop', '~> 1.3'
   s.dependency 'FirebaseCore', '~> 6.1'
-  s.dependency 'Protobuf', '~> 3.8'
+  s.dependency 'Protobuf', '~> 3.9', '>= 3.9.2'
 
   s.test_spec 'unit' do |unit_tests|
     unit_tests.source_files = 'FirebaseABTesting/Tests/Unit/*.[mh]'

FirebaseFirestore.podspec

@@ -51,7 +51,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
   s.dependency 'FirebaseCore', '~> 6.2'
   s.dependency 'gRPC-C++', '0.0.9'
   s.dependency 'leveldb-library', '~> 1.22'
-  s.dependency 'Protobuf', '~> 3.1'
+  s.dependency 'Protobuf', '~> 3.9', '>= 3.9.2'
   s.dependency 'nanopb', '~> 0.3.901'
 
   s.ios.frameworks = 'MobileCoreServices', 'SystemConfiguration'

FirebaseMessaging.podspec

@@ -49,7 +49,7 @@ device, and it is completely free.
   s.dependency 'GoogleUtilities/Reachability', '~> 6.2'
   s.dependency 'GoogleUtilities/Environment', '~> 6.2'
   s.dependency 'GoogleUtilities/UserDefaults', '~> 6.2'
-  s.dependency 'Protobuf', '~> 3.1'
+  s.dependency 'Protobuf', '~> 3.9', '>= 3.9.2'
 
   s.test_spec 'unit' do |unit_tests|
     unit_tests.source_files = 'Example/Messaging/Tests/*.{m,h,swift}'

FirebaseRemoteConfig.podspec

@@ -44,7 +44,7 @@ app update.
   s.dependency 'FirebaseInstanceID', '~> 4.2'
   s.dependency 'GoogleUtilities/Environment', '~> 6.2'
   s.dependency 'GoogleUtilities/NSData+zlib', '~> 6.2'
-  s.dependency 'Protobuf', '~> 3.9'
+  s.dependency 'Protobuf', '~> 3.9', '>= 3.9.2'
 
   s.test_spec 'unit' do |unit_tests|
     # TODO(dmandar) - Update or delete the commented files.