Actually use symfony matrix in tests (#97)

* Allow matthiasnoback/symfony-dependency-injection-test v5 for full symfony7 compatibility in dev
* Use symfony flex for ensuring symfony version in test matrix
* Exclude Symfony7 + php8.1 combo from the matrix as it is inherently incompatible
* update versions of standard github actions (checkout+cache)

Add return type required by ConfigurationInterface in Symfony 7 (#95)

supported versions (#94)

* supported versions
    - drop support for unmaintained versions
    - add support for php8.2/8.3 and symfony 7.0
* update copyrights
* run static analysis on latest stable version

Merge pull request #90 from lunetics/symfony_6

Symfony 6

gap detection can be defined in central projection config

-

- changing description of migration from 0.8.0

upgrade

- adding PHP8 support
- dropping unsupported versions
- upgrading to phpunit v9
- removing dependency for phpunit v7 due its dependency to php7.1
- running tests and code analysis on PHP8
- symfony/flex changed default branch name