Extend `ApplicationSettings` to support `default_branch_name`

app/controllers/admin/application_settings_controller.rb

@@ -227,6 +227,7 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
       :raw_blob_request_limit,
       :namespace_storage_size_limit,
       :issues_create_limit,
+      :default_branch_name,
       disabled_oauth_sign_in_sources: [],
       import_sources: [],
       repository_storages: [],

app/helpers/application_settings_helper.rb

@@ -190,6 +190,7 @@ module ApplicationSettingsHelper
       :container_expiration_policies_enable_historic_entries,
       :container_registry_token_expire_delay,
       :default_artifacts_expire_in,
+      :default_branch_name,
       :default_branch_protection,
       :default_ci_config_path,
       :default_group_visibility,

app/models/application_setting_implementation.rb

@@ -50,6 +50,7 @@ module ApplicationSettingImplementation
         default_artifacts_expire_in: '30 days',
         default_branch_protection: Settings.gitlab['default_branch_protection'],
         default_ci_config_path: nil,
+        default_branch_name: nil,
         default_group_visibility: Settings.gitlab.default_projects_features['visibility_level'],
         default_project_creation: Settings.gitlab['default_project_creation'],
         default_project_visibility: Settings.gitlab.default_projects_features['visibility_level'],

changelogs/unreleased/225258-extend-applicationsettings-to-support-default_branch_name.yml

+---
+title: Add default_branch_name to ApplicationSettings visible attrs
+merge_request: 35681
+author:
+type: other

spec/controllers/admin/application_settings_controller_spec.rb

@@ -128,6 +128,13 @@ RSpec.describe Admin::ApplicationSettingsController do
       expect(ApplicationSetting.current.repository_storages_weighted_default).to eq(75)
     end
 
+    it "updates default_branch_name setting" do
+      put :update, params: { application_setting: { default_branch_name: "example_branch_name" } }
+
+      expect(response).to redirect_to(general_admin_application_settings_path)
+      expect(ApplicationSetting.current.default_branch_name).to eq("example_branch_name")
+    end
+
     context 'external policy classification settings' do
       let(:settings) do
         {