Support PHP8 and Guzzle 7.X

**Is your feature request related to a problem? Please describe.**
Support PHP8 since it has been out for a while now and composer.json is locked to >=7.1 && <=7.4

**Describe the solution you'd like**
PHP 8 support and Guzzle 7.x.x

**Describe alternatives you've considered**
Opt-out of the SDK and using raw requests

