Use PHP8 for the PHP8 CI

[bobdenotter]

In https://github.com/bolt/project/blob/php8/.github/workflows/ci.yml I've commented out the first job, because I couldn't figure out how to make it use PHP8 instead of PHP7.4.

The php8 branch obvs needs to run on PHP8 :-)

@toofff Do you know how to fix this?

[toofff]

I look this week 😉

[toofff]

@bobdenotter to fix the CI, I'll have to upgrade the docker to PHP 8.0. Is it good for you?

[github-actions]

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