update CocoaPods integration test

IntegrationTesting/CocoapodsIntegrationTest/TestEnvironments/Cocoapods_multiprojects_frameworks/Podfile

@@ -3,7 +3,7 @@ source 'https://github.com/firebase/SpecsStaging.git'
 source 'https://cdn.cocoapods.org/'
 
 # Uncomment the next line to define a global platform for your project
-platform :ios, '11.0'
+platform :ios, '13.0'
 
 target 'CocoapodsIntegrationTest' do
   # Comment the next line if you don't want to use dynamic frameworks