Updates dependencies for security and PHP 8 compatibility

[calebtr-metro]

I needed to test this library on PHP 8, which requires an update to guzzle 7 and composer reported a conflict between phpunit and guzzle. I saw it fit to add the latest secure releases of Guzzle 6 and 7 also.

PHPunit tests pass, excepting MailchimpTest::testVersion() and MailchimpConnectedSitesTest which has not tests. I also ran the basic functional test as recommended in the readme.