fix: Add nkf and base64 to Gemfile for Ruby 3.4 compatibility (#55346)

Summary:
Ruby 3.4 removed 'kconv' (which requires 'nkf') and 'base64' from the standard library. This commit adds them to the Gemfiles to fix 'pod install' failures on Ruby 3.4 environments.

## Changelog:
[iOS][Fixed] - Add `nkf` and `base64` as ruby dependencies

Pull Request resolved: https://github.com/facebook/react-native/pull/55346

Reviewed By: cortinico

Differential Revision: D92147798

Pulled By: cipolleschi

fbshipit-source-id: ac06369593df2b892d343f45d3c43795c079a461