Doctrine ORM entities cannot be final or contain final methods

[mhujer]

See https://www.doctrine-project.org/projects/doctrine-orm/en/2.6/reference/architecture.html#entities

• Depends on fix build against latest PHPStan master #83 which should be merged first
• I'm not sure about the rules naming convention - should the rule name contain "must not" or "cannot" or nothing?

[ondrejmirtes]

Superseded by #143 :)