Run CI on PHP 8

[NanoSector]

With the increase in test coverage and the already decent coverage on the other components, it might be beneficial to additionally test on Windows.

Also with the release of PHP 8, it should be tested there.

While I do like Scrutinizer, the CI pipeline might be significantly simplified (and sped up?) if GitHub Actions is used. This might be worth looking into. Most functionality it provides can be replicated with common Composer packages and utilities.

[NanoSector]

Looks like #173 has shown that GitHub Actions is really, really slow. And it doesn't even reasonably do code analysis and coverage reports.