Update to shakapacker 9.3.0 release

Switch from using the early-hints branch to the official 9.3.0 release:
- Updated Gemfile to use shakapacker 9.3.0 (from GitHub branch)
- Updated package.json to use shakapacker 9.3.0 (from npm)
- Removed postinstall build script (no longer needed with npm release)
- Updated Gemfile.lock and yarn.lock

The early hints feature is now officially available in shakapacker 9.3.0!

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

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

Author: justin808