Remove the transport client #42638
Description
Before 5.0, the transport client (or "java client") was the only official way from java to connect to an Elasticsearch cluster. In 5.0, the low level rest client was added, in 6.0 the high level rest client, and in 7.0 the HLRC was made feature complete (most apis are implemented). Also in 7.0, the transport client was deprecated, in favor of the high level rest client.
This is a meta issue to track removal of the transport client from 8.0. In addition to the actual removal, there are several cleanups that become possible like removing many of the request builder classes, as most are not used for internal node-to-node requests.