Merge pull request thephpleague#24 from PatronBase/guzzle-psr-v2-support

Add support for GuzzleHttp/Psr7 v2.0+

Bump phpunit to .14 for PHP 8.1

Enable Mock strategy for HttpRequests

Update composer.json

Update GatewayTestCase.php

uniqid() more entropy

Update composer.json

Allow guzzle6 v2

permit higher phpunit to be used. increases compatibility with new ph…

…p versions (thephpleague#13)

Bump mock-client to always support getLastRequest