refactor(guzzle): upgrade to version 7.5

[filipcujanovic]

Description

• Upgrading guzzle version to 7.5 and updating code for the new version.
• Updating the PHP version requirement.

[muskie9]

Noting here that I came across an error at FoxyClient::368 as it attempts to convert GuzzleHttp\\Psr7\\Response to a string. I'm wondering if this would be better as $response->getReasonPhrase()?

Happy to open a PR against the polish/upgrade-guzzle-7 branch if needed.

[adamjudd]

Thanks for doing up that PR @muskie9