Kong Get Started Guide: Validations for proxy caching and load balancing

The current validations for proxy caching and load balancing on the Kong Get Started guide use the old format.

- Update the validation examples so they use `{% validation request-check %}`.
- Preferrably update the load balancing validation so the user doesn't have to run it multiple times. I'd prefer to see something that runs it multiple times for them. Ex. ChatGPT helped me put this together, but I think it needs tweaking:
```
for i in {1..4}; do curl -s http://localhost:8000/load-balance/headers \
-H 'apikey:top-secret-key' | grep -i -A1 '"host"'
sleep 2
done
```