Update Code to newer standards, requiring php 8.1 and fix deprecations

[AnnaDamm]

fixes #11

more details, see CHANGELOG.md

[simensen]

@AnnaDamm Thank you for taking the time to make this change! Are you actively using this in any projects? I'd love to see where it's being used. :)

Since most of the files showed up as deleted and recreated (ugh, git, right?), it is hard for me to evaluate what's changed here. Nothing looks off at first glance, but I'll need some time to consider this.

In the meantime, can you also add a change to composer.json to add an additional branch alias? I think these changes warrant a 2.x release. How do you feel about that?

"dev-main": "2.0.x-dev"

[simensen]

Oh, to be clear, the main branch does not exist yet. If I get to the point where I merge this, this branch will become main.

[AnnaDamm]

@simensen Thanks for looking at it. They show up as changed because I moved the files according to psr-0 to psr-4.
Sorry, I didn't consider it showing up as deleted/created in github, or i would have done it in a separated commit for easier review.

As to your question of where we use it: our company uses an hr tool with a REST-API that is using HAWK as their authentication method. We consume this API using your library here. it is also mentioned in their "how to use this rest api" documentation for php: https://documenter.getpostman.com/view/799228/absenceio-api-documentation/2Fwbis#authentication