Add Rails 7.1 compatibility

I'm seeing errors like this when trying to update from Rails 7 to 7.1.

```
Zeitwerk::NameError: expected file /gems/ruby/3.1.0/gems/solidus_user_roles-2.0.1/app/overrides/user_sub_menu.rb to define constant UserSubMenu, but didn't
      raise Zeitwerk::NameError.new(msg, cref.last)
      ^^^^^
```