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.

config/locales/server.en.yml

@@ -2725,7 +2725,6 @@ en:
     experimental_topics_filter: "EXPERIMENTAL: Enables the experimental topics filter route at /filter"
     enable_experimental_lightbox: "EXPERIMENTAL: Replace the default image lightbox with the revamped design."
     enable_experimental_bookmark_redesign_groups: "EXPERIMENTAL: Show a quick access menu for bookmarks on posts and a new redesigned modal"
-    experimental_redesigned_about_page_groups: "EXPERIMENTAL: Enable the redesigned /about page for specific groups."
     experimental_glimmer_topic_list_groups: "EXPERIMENTAL: Enable the new 'glimmer' topic list implementation. This implementation is under active development, and is not intended for production use. Do not develop themes/plugins against it until the implementation is finalized and announced."
     experimental_form_templates: "EXPERIMENTAL: Enable the form templates feature. <b>After enabled,</b> manage the templates at <a href='%{base_path}/admin/customize/form-templates'>Customize / Templates</a>."
     admin_sidebar_enabled_groups: "Enable sidebar navigation for the admin UI for the specified groups, which replaces the top-level admin navigation buttons."

config/site_settings.yml

@@ -2465,10 +2465,6 @@ developer:
     default: ""
     client: true
     hidden: true
-  experimental_redesigned_about_page_groups:
-    default: ""
-    type: group_list
-    allow_any: false
   custom_flags_limit:
     default: 50
     hidden: true