Add GitHub workflow

[kudashevs]

Hello,

I would like to propose a little improvement to CI/CD workflow. This is a PR with Github Actions file which initiates a full test run on a push or a pull request. To make tests successfully run, I had to change two things:

• add the minimal spatie/array-to-xml because version 3 works only with PHP8
• change the minimal PHP version from 7.0 to 7.1

PS I personally think, that even 7.1 is quite outdated and I would have changed the minimal version to 7.3 or 7.4 (depending on your preferences).