Upgrade Ruby from 3.3.6 to 3.4.1

Updated Ruby to the latest stable 3.4.x release for improved performance
and security updates.

Changes:
- Updated .ruby-version to 3.4.1
- Updated Gemfile ruby version requirement to 3.4.1
- Updated README.md to reflect Ruby 3.4.1 requirement
- Updated Dockerfile from ruby:2.6.5 to ruby:3.4.1
- Removed outdated bundler version constraint in Dockerfile

Tested:
- Bundle install successful with Ruby 3.4.1
- Test suite runs successfully (46 examples, 1 pre-existing failure)
- Rails server starts and responds correctly

Fixes #490

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

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

README.md

@@ -10,7 +10,7 @@ RailsGoat is an intentionally insecure Rails application designed to teach web a
 - How to identify vulnerabilities through code review and testing
 - How to implement proper security controls and remediation strategies
 
-**Current Version:** Rails 8.0 with Ruby 3.3.6
+**Current Version:** Rails 8.0 with Ruby 3.4.1
 
 ## Vulnerabilities Included
 
@@ -22,7 +22,7 @@ For a complete list of vulnerabilities with detailed explanations and tutorials,
 
 ### Prerequisites
 
-- Ruby 3.3.6
+- Ruby 3.4.1
 - Git
 - SQLite3 (included by default)
 - MySQL (optional, required for certain SQL injection demos)