Add a feature flag to control when we could prevent users to login

We want to avoid preventing legit users from logging in.
This MR adds a feature flag to control when the login can be prevented.

Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84971
EE: true

config/feature_flags/development/arkose_labs_prevent_login.yml

+---
+name: arkose_labs_prevent_login
+introduced_by_url:
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/358838
+milestone: '14.10'
+type: development
+group: group::antiabuse
+default_enabled: false

ee/app/services/arkose/user_verification_service.rb

@@ -122,6 +122,8 @@ module Arkose
     end
 
     def low_risk?(response)
+      return true unless Feature.enabled?(:arkose_labs_prevent_login, default_enabled: :yaml)
+
       risk_band = risk_band(response)
       risk_band.present? ? risk_band != 'High' : true
     end

ee/spec/services/arkose/user_verification_service_spec.rb

@@ -21,6 +21,7 @@ RSpec.describe Arkose::UserVerificationService do
       end
     end
 
+    context 'when feature arkose_labs_prevent_login is enabled' do
       context 'when the user solved the challenge' do
         context 'when the risk score is not high' do
           let(:arkose_ec_response) { Gitlab::Json.parse(File.read(Rails.root.join('ee/spec/fixtures/arkose/successfully_solved_ec_response.json'))) }
@@ -102,6 +103,7 @@ RSpec.describe Arkose::UserVerificationService do
           end
         end
       end
+    end
 
     context 'when an error occurs during the Arkose request' do
       it 'returns true' do
@@ -109,5 +111,20 @@ RSpec.describe Arkose::UserVerificationService do
         expect(subject).to be_truthy
       end
     end
+
+    context 'when feature arkose_labs_prevent_login is disabled' do
+      before do
+        stub_feature_flags(arkose_labs_prevent_login: false)
+      end
+
+      context 'when the risk score is high' do
+        let(:arkose_ec_response) { Gitlab::Json.parse(File.read(Rails.root.join('ee/spec/fixtures/arkose/successfully_solved_ec_response_high_risk.json'))) }
+
+        it 'returns true' do
+          allow(Gitlab::HTTP).to receive(:perform_request).and_return(response)
+          expect(subject).to be_truthy
+        end
+      end
+    end
   end
 end