DEV: Convert allow_uploaded_avatars to groups (#24810)

This change converts the allow_uploaded_avatars site setting to uploaded_avatars_allowed_groups.

See: https://meta.discourse.org/t/283408

Hides the old setting
Adds the new site setting
Adds a deprecation warning
Updates to use the new setting
Adds a migration to fill in the new setting if the old setting was changed
Adds an entry to the site_setting.keywords section
Updates tests to account for the new change
After a couple of months, we will remove the allow_uploaded_avatars setting entirely.

Internal ref: /t/117248

spec/system/user_page/user_preferences_account_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 describe "User preferences for Account", type: :system do
-  fab!(:user)
+  fab!(:user) { Fabricate(:user, refresh_auto_groups: true) }
   let(:user_account_preferences_page) { PageObjects::Pages::UserPreferencesAccount.new }
   let(:avatar_selector_modal) { PageObjects::Modals::AvatarSelector.new }
   before { sign_in(user) }