feat: Upgrade to containerd v2.1.4 with proper API compatibility

Upgrades containerd from v1.7.27 to v2.1.4 with full API compatibility:

Containerd v2.1.4 Upgrade:
- Updated go.mod to use containerd/v2 v2.1.4 (stable release)
- Updated imports to use v2 API paths:
  - github.com/containerd/containerd/v2/client
  - github.com/containerd/containerd/v2/contrib/seccomp
  - github.com/containerd/containerd/v2/pkg/namespaces
  - github.com/containerd/containerd/v2/pkg/oci
- Updated client initialization and method calls for v2 API
- Fixed test files to use correct v2 types (containers.Container)

Root Cause Analysis:
- Previous incompatibility was due to using containerd/v2 v2.0.0-rc.4
- RC version had incomplete errdefs package implementation
- v2.1.4 is stable release with proper errdefs.FromGRPC support
- v2 API provides better performance and security features

Benefits of v2.1.4:
- Stable API with proper error handling
- Improved performance and resource management
- Enhanced security features and hardening
- Better compatibility with modern container runtimes
- Full errdefs package support

Files modified:
- go.mod - Updated to containerd/v2 v2.1.4
- internal/sandbox/containerd.go - Updated to v2 API
- internal/sandbox/containerd_hardening_test.go - Updated imports
- tests/security/hardened_container_test.go - Updated imports

Build Status: ✅ Working
Tests: ✅ Passing
Web Server: ✅ Working
API Endpoints: ✅ Working