[iOS] Add nkf gem to Gemfile

[cipolleschi]

Summary:

Ruby 3.4.x is not shipping 'nkf' anymore. Some users might encounter some issues without this Gem

Changelog:

[iOS][Added] - Add nkf gem to Gemfi

Test Plan:

With Ruby 3.4.x:

npx @react-native-community/cli init Next --version next --skip-install
cd Next
yarn
cd ios
bundle install
bundle exec pod install

It fails without the gem. It succeeds otherwise.