[REQ][PHP] Bump guzzle to version 7

Created by: toymachiner62

I have built a lib for others to use and it's using guzzle version 6.x b/c that's what openapi-generator uses. When other devs are consuming my project they are unable to use guzzle 7 since version 6 is what's being used in my lib generated from openapi-generator.

I come from node.js for the 8 years so i'm used to all of a projects dependencies using their own versions, but I guess php land is not like this.

Can you bump guzzle to version to 7? Or is there another way to handle this dependency of a dependency issue that i'm unaware of?