feat!: modernize plugin for Gatsby 5 (v2.0.0)

BREAKING CHANGE: Major version update requiring Gatsby 5+ and Node 18+

- Replace axios with native fetch API
- Remove gatsby-node-helpers, use native Gatsby APIs
- Update all dependencies to match Gatsby's versions
- Modernize error handling with reporter API
- Restore conflictFieldPrefix (gh_ prefix) for backward compatibility
- Add migration guide and breaking changes documentation