[ToDo] PhpStan level 6 + Rector for assistance

[pk16011990]

Q	A
Description, reason for the PR	I want a higher PhpStan level, and I do not want to update all files manually
New feature	Yes/No (probably yes)
BC breaks	Yes
Fixes issues	...
Have you read and signed our License Agreement for contributions?	Yes

It needs a repeated execution of Rector because each run adds more context for other rules. Except for the \Shopsys\FrameworkBundle\Component\Rector\GuessReturnTypeByImplementationRector because It is broken for repeated executions

Todo:

• Coding standards
• Applying not only to shopsys/framework and project-base
• Codestyle in Rector rules
• Decide how to add Rector to a project and where (coding-standards?)
• fix remaining PhpStan errors (299)
• fix everything that has been broken (build has to be successful)
• upgrade notes