warnings() should not include warnings not originating from Elasticsearch

[rudolf]

The elasticsearch-js client exposes Elasticsearch deprecations through the warnings getter:

https://github.com/elastic/elastic-transport-js/blob/main/src/Transport.ts#L333

However, this returns all the warning headers which could include warnings from intermediate proxies.

Given the htttp specification for warning headers https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46

We should be able to include only warning headers from Elasticsearch with a pattern like /\d\d\d Elasticsearch-/