Upgrade to Ruby 3.3.6 and Rails 8.0.4

This major upgrade brings RailsGoat up to date with the latest versions:
- Ruby 2.6.5 → 3.3.6
- Rails 6.0.0 → 8.0.4

## Key Changes

### Dependencies
- Upgraded all gems to Rails 8-compatible versions
- Removed deprecated gems: therubyracer, coffee-rails, poltergeist,
  travis-lint, rails-perftest, unicorn, powder, rubocop-github
- Updated puma to 6.6.1, sqlite3 to 2.8.1, rspec-rails to 8.0.2
- Added modern Rails 8 features: importmap-rails, stimulus-rails, turbo-rails
- Replaced poltergeist with selenium-webdriver for integration tests

### Code Changes
- Converted CoffeeScript files to plain JavaScript
- Updated test configuration to use Selenium headless driver
- Updated database schema to Rails 8 format

## Testing
- Application starts successfully and responds to requests
- Test suite runs with 23 examples (14 intentional vulnerability failures)
- Database migrations applied successfully

## Notes
This upgrade maintains all intentional security vulnerabilities that make
RailsGoat an effective training tool. The failing tests are expected and
demonstrate the vulnerabilities the application is designed to teach.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

spec/spec_helper.rb

@@ -9,7 +9,7 @@ SimpleCov.start if ENV["COVERAGE"]
 require File.expand_path("../../config/environment", __FILE__)
 require "rspec/rails"
 require "capybara/rails"
-require "capybara/poltergeist"
+require "selenium-webdriver"
 require "database_cleaner"
 
 # Requires supporting ruby files with custom matchers and macros, etc,
@@ -61,6 +61,6 @@ RSpec.configure do |config|
   config.infer_spec_type_from_file_location!
 end
 
-Capybara.javascript_driver = :poltergeist
+Capybara.javascript_driver = :selenium_headless
 
 DatabaseCleaner.strategy = :truncation