DEV: Refactor some core services

Extracted from https://github.com/discourse/discourse/pull/29129.

This patch makes the code more compliant with the upcoming service docs
best practices.

app/controllers/admin/config/about_controller.rb

@@ -36,8 +36,8 @@ class Admin::Config::AboutController < Admin::AdminController
     end
 
     settings_map.each do |name, value|
-      UpdateSiteSetting.call(
-        guardian: guardian,
+      SiteSetting::Update.call(
+        guardian:,
         setting_name: name,
         new_value: value,
         allow_changing_hidden: %i[

app/controllers/admin/site_settings_controller.rb

@@ -39,10 +39,10 @@ class Admin::SiteSettingsController < Admin::AdminController
 
     previous_value = value_or_default(SiteSetting.get(id)) if update_existing_users
 
-    UpdateSiteSetting.call(service_params.merge(setting_name: id, new_value: value)) do
+    SiteSetting::Update.call(service_params.merge(setting_name: id, new_value: value)) do
       on_success do
         if update_existing_users
-          SiteSettingUpdateExistingUsers.call(id, result.new_value, previous_value)
+          SiteSettingUpdateExistingUsers.call(id, result.contract.new_value, previous_value)
         end
         render body: nil
       end

app/controllers/admin/users_controller.rb

@@ -124,7 +124,7 @@ class Admin::UsersController < Admin::StaffController
       on_success do
         render_json_dump(
           suspension: {
-            suspend_reason: result.reason,
+            suspend_reason: result.contract.reason,
             full_suspend_reason: result.full_reason,
             suspended_till: result.user.suspended_till,
             suspended_at: result.user.suspended_at,