chore: remove duplicate dependency-update workflow

Remove custom dependency-update.yml workflow as Dependabot already provides
the same functionality with better features including:
- Automated PR creation
- Dependency grouping
- Multi-ecosystem support (Gradle + GitHub Actions)
- Native GitHub integration

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

Co-Authored-By: Claude <[email protected]>

Author: graycreate