Feature/modernize

[Chris53897]

Hi. I modernized some things, hope you like it.

• Add github actions
• Update readme
• Update dependencies
• Remove deprecations
• Fixes for --prefer-lowest (see last 3 entries in composer.json require-dev section.)
• Set PHP to 7.2 (there are no infos about the desired language level) Please let me know if you wish an other php persion as lowest level.

I need to add 2 tests only to run on php > 8.
These are because of the newly added php8 attributes.
I am not sure if the php8 attributes commit is running on < php 8.
Or if this is just an problem of the tests.

Please have a look at Tests/Functional/ValidatorAnnotationTest.php if my changes are correct.

see Test-Run

[ddeboer]

Thanks! Implemented in #30.