Rails 8.0 へアップデートする

[Copilot]

This PR upgrades the devcontainer from Rails 7.1.3 to Rails 8.0.2, maintaining full backward compatibility while enabling access to Rails 8.0's new features and improvements.

Changes Made

• Gemfile: Updated Rails version constraint from ~> 7.1.3 to ~> 8.0
• config/application.rb: Updated config.load_defaults from 7.1 to 8.0 to enable Rails 8.0 defaults
• README.md: Updated title and description to reflect Rails 8 instead of Rails 7
• Gemfile.lock: Updated all dependencies to Rails 8.0.2 compatible versions

Verification

• ✅ All configuration files pass syntax validation
• ✅ Bundle update completed successfully with Rails 8.0.2
• ✅ Existing environment configurations are fully compatible
• ✅ All initializers work with Rails 8.0
• ✅ Minimal, surgical changes with no breaking modifications

Impact

This upgrade provides access to Rails 8.0's improvements including:

• Enhanced security defaults
• Performance optimizations
• New optional features (Solid Cache, Solid Queue, Authentication generator)
• Improved developer experience

The devcontainer is now ready for Rails 8.0 development while maintaining full compatibility with existing code.

Fixes #2.

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