update guzzle requirements

[vv12131415]

Since guzzle 7 is out I think it will be cool to support it here, in this package.

[dmyers]

This is required in order to upgrade to Laravel 8.

cc @theiconic

[andrekutianski]

Is there any chance that this project doesn't depend on guzzle in any way?

[vv12131415]

it's already supports guzzle 7 as dependency

@andrekutianski