Remove legacy CircleCI configuration in favor of GitHub Actions

- CircleCI config was marked as compatibility-only during transition
- GitHub Actions provides comprehensive CI/CD with:
  * Spring Boot 3.5 and Java 21 support
  * Multi-job pipeline (test, build, code-quality, security)
  * MySQL service integration
  * OWASP dependency scanning
  * Trivy vulnerability scanning
  * TruffleHog secrets detection
  * SonarCloud integration with secure token management
  * Artifact uploads and test reporting

- CircleCI config was incomplete with most features disabled
- Removed security risk from legacy hardcoded tokens
- Standardizes on modern GitHub-native CI/CD approach

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

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

Author: dfcoffin