[action][get_push_certificate] remove all instances of is_string in…

… options and use `type` (#18881)

fastlane/lib/fastlane/actions/get_push_certificate.rb

@@ -55,7 +55,7 @@ def self.available_options
         @options << FastlaneCore::ConfigItem.new(key: :new_profile,
                                      description: "Block that is called if there is a new profile",
                                      optional: true,
-                                     is_string: false)
+                                     type: :string_callback)
         @options
       end