x-elastic-client-meta is not properly reporting when Undici is being used

The client should be modifying the `x-elastic-client-meta` header to distinguish when `UndiciConnection` or `HttpConnection` is being used. Right now it [hard codes](https://github.com/elastic/elasticsearch-js/blob/84ab2a787d0922ae3d317c9f87c987339595b18c/src/client.ts#L211) `hc`, and I verified in our telemetry that there is no way to distinguish Undici requests from `http` requests.