FIX: rename notify_about_flags_after to notify_about_reviewable_item_after (#21320)

Change name and description for SiteSetting to make it easier to understand.

db/migrate/20230501022508_rename_notify_about_flags_after_site_setting.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class RenameNotifyAboutFlagsAfterSiteSetting < ActiveRecord::Migration[7.0]
+  def up
+    execute "UPDATE site_settings SET name = 'notify_about_reviewable_item_after' WHERE name = 'notify_about_flags_after'"
+  end
+
+  def down
+    execute "UPDATE site_settings SET name = 'notify_about_flags_after' WHERE name = 'notify_about_reviewable_item_after'"
+  end
+end