Update for phpunit

[esurov]

This allows to update phpunit/phpunit with it's dependencies to 8 to 9 and correctly work with PHP8.*

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, ...)

It bumps the version of phpunit to make coverage work on PHP 8

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

Currently there is a problem described in the issue #30 https://github.com/inpsyde/modularity/issues/30

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

This PR fixed this issue.

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

No, it does not.

Other information:

[Chrico]

Right now we have in our composer.json PHP 7.2 as minimum version. PHPUnit 9.x requires PHP 7.3 as min. So you probably need to change the constraint to "^8.5.21 || "^9.6.7"" to support all PHP versions.

[esurov]

@Chrico Good point. I have pushed another commit there.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.98 :warning:

Comparison is base (eb14a9b) 96.86% compared to head (c52eb0b) 95.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #31 +/- ## ============================================ - Coverage 96.86% 95.88% -0.98% + Complexity 177 169 -8 ============================================ Files 9 9 Lines 510 462 -48 ============================================ - Hits 494 443 -51 - Misses 16 19 +3 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `95.88% <ø> (-0.98%)` | :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. [see 9 files with indirect coverage changes](https://app.codecov.io/gh/inpsyde/modularity/pull/31/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inpsyde)

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