feature #5735 [Contributing][Code] do not distinguish regular classes and API classes (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Contributing][Code] do not distinguish regular classes and API classes

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | kind of (symfony/symfony#15977, symfony/symfony#15979)
| Applies to    | all
| Fixed tickets |

Commits
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9794d58 do not distinguish regular classes and API classes

Author: weaverryan