DEV: Convert min_trust_to_post_links to groups (#25298)

We're changing the implementation of trust levels to use groups. Part of this is to have site settings that reference trust levels use groups instead. It converts the min_trust_to_post_links  site setting to post_links_allowed_groups.

This isn't used by any of our plugins or themes, so very little fallout.

spec/requests/admin/users_controller_spec.rb

@@ -1181,7 +1181,7 @@ RSpec.describe Admin::UsersController do
   end
 
   describe "#destroy" do
-    fab!(:delete_me) { Fabricate(:user) }
+    fab!(:delete_me) { Fabricate(:user, refresh_auto_groups: true) }
 
     shared_examples "user deletion possible" do
       it "returns a 403 if the user doesn't exist" do