Update shakapacker gem version to 9.3.0 in Pro Gemfiles

Updated shakapacker gem version from 8.0.0 to 9.3.0 in Pro Gemfiles to match
the package.json changes, fixing the version mismatch error:

  "Shakapacker gem and node package versions do not match
   Detected: 9.3.0
        gem: 8.0.0"

Changed in:
- react_on_rails_pro/Gemfile.development_dependencies (8.0.0 → 9.3.0)
- react_on_rails_pro/spec/execjs-compatible-dummy/Gemfile (8.0 → 9.3.0)

This ensures gem and npm package versions match across all Pro dummy apps.

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

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

Author: justin808

react_on_rails_pro/spec/execjs-compatible-dummy/Gemfile

@@ -61,7 +61,7 @@ group :test do
   gem "selenium-webdriver"
 end
 
-gem "shakapacker", "= 8.0"
+gem "shakapacker", "= 9.3.0"
 
 gem "react_on_rails", path: "../../.."
 gem "react_on_rails_pro", path: "../.."