PHPStan level 6

[franmomu]

It would be nice to reach level 6 for 2.3, but that would require quite some work, apart from #2383, there are two more checks to reach that level:

• checkGenericClassInNonGenericObjectType (198 errors): This one, every generic class should have defined its type/s.
• checkMissingIterableValueType (498 errors): This one means that every iterable should define the type of the items.

For the first one, we should probably bump to PHPStan 1 (to be able to use array as template type), but we'll see. I'll try to provide PRs with small changes when I find some time.

[stale]

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