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>

db/schema.rb

@@ -1,18 +1,16 @@
-# frozen_string_literal: true
 # This file is auto-generated from the current state of the database. Instead
 # of editing this file, please use the migrations feature of Active Record to
 # incrementally modify your database, and then regenerate this schema definition.
 #
-# Note that this schema.rb definition is the authoritative source for your
-# database schema. If you need to create the application database on another
-# system, you should be using db:schema:load, not running all the migrations
-# from scratch. The latter is a flawed and unsustainable approach (the more migrations
-# you'll amass, the slower it'll run and the greater likelihood for issues).
+# This file is the source Rails uses to define your schema when running `bin/rails
+# db:schema:load`. When creating a new database, `bin/rails db:schema:load` tends to
+# be faster and is potentially less error prone than running all of your
+# migrations from scratch. Old migrations may fail to apply correctly if those
+# migrations use external dependencies or application code.
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171007010129) do
-
+ActiveRecord::Schema[8.0].define(version: 2017_10_07_010129) do
   create_table "analytics", force: :cascade do |t|
     t.string "ip_address"
     t.string "referrer"
@@ -113,5 +111,4 @@ ActiveRecord::Schema.define(version: 20171007010129) do
     t.datetime "updated_at"
     t.binary "encrypted_ssn"
   end
-
 end