Update security warning to use repository's standard caution note format

Co-authored-by: sgbaird <[email protected]>

Author: Copilot

docs/courses/hello-world/1.4.2-non-micropython-device-version.ipynb

@@ -77,7 +77,7 @@
         "2. **Set your unique device ID** in the `COURSE_ID` variable\n",
         "3. **Ensure your device ID matches** what you're using in the companion notebook\n",
         "\n",
-        "⚠️ **Security Warning:** The credentials shown below are public test credentials for educational purposes only. In production environments, never hardcode credentials directly in your code or expose them in plain text. Instead, use environment variables, secure configuration files, or cloud secret management services."
+        "> **_CAUTION:_** The credentials shown below are public test credentials for educational purposes only. In production environments, never hardcode credentials directly in your code or expose them in plain text. Instead, use environment variables, secure configuration files, or cloud secret management services."
       ],
       "metadata": {
         "id": "mqttSetupNote"