Update requirements from php-http to psr

[Taluu]

As PSR-18 was passed some months ago, maybe instead of requiring php-http/client-implementation, requiring psr/http-client-implementation (not sure of the name) would be more appropriate ?

[Nyholm]

That would be a BC break. However, all clients implementing psr/http-client-implementation should also (easily) implement php-http/httplug:2.0. We will support HTTPlug 2.0 in #802

[acrobat]

Closing this issue as @Nyholm stated, all clients implement the psr client implementation and since #802 (v2.12.0) we have support for httplug 2.0