change: update the default rate limiting rejection code from 503 to 429

### Current Behavior

Previous discussion: https://www.mail-archive.com/dev@apisix.apache.org/msg05298.html

Notably:
> The rate-limit plugin currently defaults to 503 HTTP response code for requests exceeding the configured rate limit. HTTP `429 Too Many Requests` seems to be a better fit: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/429
>
> 503 was initially used because Nginx's limit_req_status also uses 503 as the default. However, Nginx made its decision long before 429 became popular.

This isn't a bug per se, but an enhancement, which has been noted that it would be a breaking change if implemented.

Plugins in scope:
* `limit-conn`
* `limit-count`
* `limit-req`

### Steps to Reproduce

https://docs.api7.ai/apisix/getting-started/rate-limiting

### Environment

APISIX version 3.2

Start with the limit-conn, limit-count, and limit-req plugins, then read the linked rate-limiting guide and prior discussion. Done means the default rejection response for all three plugins uses HTTP 429 instead of 503, with the breaking-change impact considered.

Written by the indexing model from the issue text.