Add ostruct as an explicit dependency

[Floppy]

It's being removed from default gems in Ruby 3.5.0, and currently shows a warning when devise_zxcvbn is used:

11:05:21 rails.1              | /usr/src/app/vendor/bundle/ruby/3.3.0/gems/zeitwerk-2.7.1/lib/zeitwerk/core_ext/kernel.rb:34: warning: /usr/local/lib/ruby/3.3.0/ostruct.rb was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
11:05:21 rails.1              | You can add ostruct to your Gemfile or gemspec to silence this warning.
11:05:21 rails.1              | Also please contact the author of devise_zxcvbn-6.0.0 to request adding ostruct into its gemspec.