Update grumphp to a version compatibile with composer 2

[eileenmcnaughton]

I think this will address the dependency error in https://github.com/thephpleague/omnipay-paypal/pull/245/commits

[leith]

This will break PHP compatibility - it needs minimum PHP 7.3, which isn't even on the list of CI environments to test yet. 😓

This would be another approach: https://github.com/thephpleague/omnipay-common/pull/239

[eileenmcnaughton]

Sigh - yeah supporting 5.6 & 7.x on the same branch is of a project is becoming increasingly unfeasible

[barryvdh]

I've removed it and added Github actions tests.