Ruby on Rails: Update to use HTTParty instead of RestClient

[davidumoru]

Because

The rest-client gem used in the Kittens API lesson is in maintenance mode and hasn't received updates in years. This PR updates the lesson to use HTTParty, a modern, actively maintained gem that is widely used in the Rails ecosystem.

This PR

• Replaces global rest-client installation with bundle add httparty to add the gem to the project's Gemfile
• Changes from using IRB to Rails console for testing API requests
• Updates all RestClient.get() calls to HTTParty.get()
• Updates JSON request header syntax from accept: :json to headers: { 'Accept' => 'application/json' }
• Updates all code examples and variable names to reflect the new HTTParty syntax

Issue

Closes #30456

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[JoshDevHub]

Thank you so much! Great work 🚀