Use User#human? instead of user_type.nil?

Rails 6.1 now returns `human` string instead of `nil`
for a human User type

app/workers/update_highest_role_worker.rb

@@ -17,7 +17,7 @@ class UpdateHighestRoleWorker
 
     return unless user.present?
 
-    if user.active? && user.user_type.nil? && !user.internal?
+    if user.active? && user.human? && !user.internal?
       Users::UpdateHighestMemberRoleService.new(user).execute
     else
       UserHighestRole.where(user_id: user_id).delete_all

spec/services/users/build_service_spec.rb

@@ -179,7 +179,7 @@ RSpec.describe Users::BuildService do
             params.merge!({ user_type: :project_bot })
           end
 
-          it { expect(user.project_bot?).to be true}
+          it { expect(user.project_bot?).to be true }
         end
 
         context 'when not a project_bot' do
@@ -187,7 +187,7 @@ RSpec.describe Users::BuildService do
             params.merge!({ user_type: :alert_bot })
           end
 
-          it { expect(user.user_type).to be nil }
+          it { expect(user).to be_human }
         end
       end