Github Actions: Run composer install --no-dev with various PHP versions

[UlrichEckhardt]

Clockwork currently supports 7.1 - 8.4, so it should better work with them. This PR automates this task for every push.

[itsgoingd]

This looks like a good template for when we actually have some tests to run, but testing just installation is probably not necessary?

[UlrichEckhardt]

Yes and no: I think there was a regression some months ago, which made Clockwork uninstallable on some PHP version. That regression was quickly fixed, but this PR makes sure that it won't return. And yes, it can be the base for automated tests, of which there are none yet.

[UlrichEckhardt]

I just had another look, so let me clarify: The only (and IMHO sufficient) info on the supported PHP versions is in composer.json. These GitHub Actions just verify that this info is correct, so that you don't have to manually verify it on every change.

BTW: I do also have a feature branch with PHPUnit integration and some tests for the file storage, but never made a pull request. I'm working on that now. I'm going to use PHPUnit 10 there, which has a minimum of PHP 8.1, but that only affects the dev dependencies. This PR here can then give a proof that I didn't make Clockwork uninstallable for earlier PHP versions by accident.