Require PHP 7.4, Fix CS and QA

[gmazzap]

• PHPCS was not checked due to wrong configuration. Fix config (to latest Inpsyde CS) and the fix code
• Use typed properties when possible
• Use doc bloc consistently, delete unnecessary comments
• Update dependencies and GHA workflow (no need to support PHPUnit 8)
• Added Package::STATUS_BOOTING constant to alias Package::STATUS_MODULES_ADDED for clarity

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes/features)
• [x] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix (for PHPCS config) + maintenance update

What is the current behavior? (You can also link to an open issue here)

PHPCS did not check anything.

What is the new behavior (if this is a feature change)?

No change in behavior. PHPCS now works (besides being updated).

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.93%. Comparing base (c6855a6) to head (b7a7fe4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #48 +/- ## ============================================ - Coverage 98.97% 98.93% -0.04% - Complexity 217 234 +17 ============================================ Files 10 10 Lines 588 566 -22 ============================================ - Hits 582 560 -22 Misses 6 6 ``` | [Flag](https://app.codecov.io/gh/inpsyde/modularity/pull/48/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/inpsyde/modularity/pull/48/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde) | `98.93% <100.00%> (-0.04%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde#carryforward-flags-in-the-pull-request-comment) to find out more.

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[gmazzap]

I would like to wait for the https://github.com/inpsyde/modularity/issues/47 before merging these updates.

@Chrico ok. I thought about doing it the other way around. But it is also fine get the generic hook out first. I'll comment there to agree on the hook name so we can move that forward.

[gmazzap]

Thanks for the review and the added test @tfrommen