Support PSR-18

[Nyholm]

This will drop PHP5 and force us to release a new major version.

[kelunik]

We already require PHP 7 here.

[Nyholm]

Great. I was just copy pasting multiple issues.

I've started a project (https://github.com/orgs/php-http/projects/1) and I've also started to migrate Guzzle6 adapter. Lets see how that goes, then I'll start with all other adapters/clients.