Upgraded dependencies, composer:^2 and php:^8, mutation tests CI improvements

[Ocramius]

I was mostly annoyed by the composer install time in CI environments, so I decided to give this a shot and update to php:^8 and composer/composer:^2.

TODO:

• add CI task that performs a docker build, ensuring that our build process still runs

Blockers:

• php:~8.0.0 support jwage/changelog-generator#64
• Upgraded dependencies, dropped PHP < 7.3 jwage/changelog-generator#65
• php:~8.0.0 support phly/keep-a-changelog#96
• feat: Upgraded to PHP 8, dropped PHP <7.4, upgraded dependencies phly/keep-a-changelog#97

[geerteltink]

LGTM when the dependencies are updated. This should improve the build time a lot.

@Ocramius have you thought about building the docker container itself and push that to a registry? That way the container doesn't need to be build repeatedly and would improve speed even more. I'm not sure how that would work with actions though or if we can use GitHub registry for it. Last time I checked you could not access the GitHub registry without tokens.

[Ocramius]

Yeah, unsure about docker builds: would be totally for it if it were globally usable 🤔

[Ocramius]

Note: the added mumbojumbo around mutation-tests.yml is mostly to produce decent reports (and fast ones) for users that send patches to improve small sections of the code.

That ensures that we don't need to run the full mutation test suite against everything every time, if the diff is limited to some sources.

The output looks like this:

[Ocramius]

Just to make sure everything went smoothly, I ran this on a pre-existing tag of ocramius/LazyMap, and it works fine: https://github.com/Ocramius/LazyMap/runs/1739730760?check_suite_focus=true#step:4:12 (failure expected, since tag already exists)