FEATURE: Use new topic bulk actions menu for all sites (#28003)

This commit promotes the new topic bulk action
menu introduced in 89883b2f51
to the main method of bulk selecting and performing
actions on topics. The site setting flag gating this
feature is deleted, and the old bulk select code is
deleted as well.

The new modal shows a loading spinner while operations
are taking place, allows selecting the action from a dropdown
instead of having a 2-step modal flow,
and also supports additional options for some operations, e.g.
allowing Close silently.

config/locales/server.en.yml

@@ -2679,7 +2679,6 @@ en:
     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."
     lazy_load_categories_groups: "EXPERIMENTAL: Lazy load category information only for users of these groups. This improves performance on sites with many categories."
-    experimental_topic_bulk_actions_enabled_groups: "EXPERIMENTAL: Enable the new bulk actions dropdown."
 
     page_loading_indicator: "Configure the loading indicator which appears during page navigations within Discourse. 'Spinner' is a full page indicator. 'Slider' shows a narrow bar at the top of the screen."
     show_user_menu_avatars: "Show user avatars in the user menu"

config/site_settings.yml

@@ -2425,12 +2425,6 @@ developer:
     default: ""
     client: true
     hidden: true
-  experimental_topic_bulk_actions_enabled_groups:
-    default: ""
-    type: group_list
-    list_type: compact
-    allow_any: false
-    refresh: true
   experimental_flags_admin_page_enabled_groups:
     default: ""
     type: group_list