bump top-level podspec requirement

Author: morganchen12

FirebaseUI.podspec

@@ -7,7 +7,7 @@ Pod::Spec.new do |s|
   s.source       = { :git => 'https://github.com/firebase/FirebaseUI-iOS.git', :tag => 'v' + s.version.to_s}
   s.author       = 'Firebase'
   s.platform = :ios
-  s.ios.deployment_target = '11.0'
+  s.ios.deployment_target = '12.0'
   s.ios.framework = 'UIKit'
   s.requires_arc = true
   s.public_header_files = 'FirebaseUI.h'