[Infra] Bump podspec min. versions (#15015)

FirebaseAuthTestingSupport.podspec

@@ -17,9 +17,9 @@ Pod::Spec.new do |s|
     :tag => 'CocoaPods-' + s.version.to_s
   }
 
-  ios_deployment_target = '13.0'
+  ios_deployment_target = '15.0'
   osx_deployment_target = '10.15'
-  tvos_deployment_target = '13.0'
+  tvos_deployment_target = '15.0'
   watchos_deployment_target = '7.0'
 
   s.swift_version = '5.9'