Update Node.js requirement from 18+/16 to 20+ across documentation

Fixes Node version inconsistencies identified by CodeRabbit in PR #1860.

Changes:
- docs/introduction.md: 18+ → 20+
- docs/getting-started/quick-start.md: 18+ → 20+
- docs/deployment/heroku-deployment.md: 16.19.0 → 20.0.0

Rationale:
- React on Rails v16 requires Node 20+ for ESM module support
- Node 16 reached EOL in September 2023
- Node 18 reaches EOL in April 2025
- CI currently tests only Node 20 and 22
- Release notes specify Node 20.19.0+ or 22.12.0+ required

The tutorial.md (line 7) says "Updated for React on Rails v16" and
users following it would use Node 20+, so deployment guide should
match to avoid confusion.

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

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

Author: ihabadham

docs/deployment/heroku-deployment.md

@@ -129,7 +129,7 @@ Next, update your `package.json` to specify the version of yarn and node. Add th
 
 ```json
   "engines": {
-    "node": "16.19.0",
+    "node": "20.0.0",
     "yarn": "1.22.4"
   },
 ```

docs/getting-started/quick-start.md

@@ -12,7 +12,7 @@ Before starting, make sure you have:
 - **🚨 Shakapacker 6+** (7+ recommended for React on Rails 16)
 - **Rails 7+** application (Rails 5.2+ supported)
 - **Ruby 3.0+** (required)
-- **Node.js 18+** and **Yarn**
+- **Node.js 20+** and **Yarn**
 - **Basic familiarity** with React and Rails
 
 > 💡 **Don't have a Rails app?** Run `rails new my_react_app` first.

docs/introduction.md

@@ -104,7 +104,7 @@ Read the full **[React on Rails Doctrine](./misc/doctrine.md)** for our design p
 
 - **Rails 7+** (Rails 5.2+ supported)
 - **Ruby 3.0+**
-- **Node.js 18+**
+- **Node.js 20+**
 - **Shakapacker 6+** (7+ recommended for React on Rails v16)
 
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