Is your feature request related to a problem? Please describe.
All OpenSearch clients (rest-client., RHLC, opensearch-java) use Apache HttpComponents / Core 4.x, which are already EOLed and superseded by Apache HttpClient / Core 5.x. The migration to the Apache HttpClient / Core 5.x is a necessary undertaking (not only to benefit from HTTP/2 support but also to rely on maintanable libraries).

Describe the solution you'd like
Migrate client transports to Apache HttpClient / Core 5.x

Describe alternatives you've considered
Stay on Apache HttpComponents / Core 4.x

Additional context
See please #4257