chore: fix all phpcs and phpstan issues

[mRoca]

• fix all phpcs issues
• fix all phpstan issues
• add QA tools in dev dependencies for all developpers
• set the oldest working version of all packages in composer.json / remove the ones not compatible with the main code
• upgrade github actions ubuntu image
• add php 8.4 in tests matrix
• add the documentation about "how to run tests with the lowest allowed versions" in README.md

I will add Symfony 8 in another PR after this one's merge.

[Daniel-Marynicz]

@mRoca phpstan requires php 7.4. easiest to fix to is to drop support to php 7.3 and older versions.

[mRoca]

@mRoca phpstan requires php 7.4. easiest to fix to is to drop support to php 7.3 and older versions.

I dropped php 7.2 and 7.3, and use php 8.4 for phpstan and phpcs in CI : https://github.com/Daniel-Marynicz/BehatParallelExtension/compare/c1696c6b7b8038a6cb9dbd93d502474ec38c5190..a338157776ce39b343f42386953ac8faacb59dbd

php >= 7.4 < 8.2 will only be used for phpunit and behat