Default maintenance_mode value to false if not explicitly set

ee/app/models/ee/application_setting.rb

@@ -157,6 +157,7 @@ module EE
           geo_node_allowed_ips: '0.0.0.0/0, ::/0',
           git_two_factor_session_expiry: 15,
           lock_memberships_to_ldap: false,
+          maintenance_mode: false,
           max_personal_access_token_lifetime: nil,
           mirror_capacity_threshold: Settings.gitlab['mirror_capacity_threshold'],
           mirror_max_capacity: Settings.gitlab['mirror_max_capacity'],

ee/changelogs/unreleased/ag-fix-db-read-only-check.yml

+---
+title: Default maintenance_mode value to false if not explicitly set
+merge_request: 54428
+author:
+type: fixed

ee/spec/models/application_setting_spec.rb

@@ -782,4 +782,10 @@ RSpec.describe ApplicationSetting do
       it { is_expected.to be true }
     end
   end
+
+  describe 'maintenance mode setting' do
+    it 'defaults to false' do
+      expect(subject.maintenance_mode).to be false
+    end
+  end
 end