FEATURE: Enable the new /about page for everyone (#29390)

This commit removes the feature flag for the new /about page, enabling it for all sites, and removes the code for old the /about page.

Internal topic: t/140413.

db/migrate/20241024102733_remove_experimental_redesigned_about_page_groups_setting.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class RemoveExperimentalRedesignedAboutPageGroupsSetting < ActiveRecord::Migration[7.1]
+  def up
+    execute(<<~SQL)
+      DELETE FROM site_settings
+      WHERE name = 'experimental_redesigned_about_page_groups'
+    SQL
+  end
+
+  def down
+    raise IrreversibleMigration.new
+  end
+end