Add missing return types for classes implementing core interfaces

[ovidiuenache]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	yes (for PHP < 7.4.0)

Summary

I noticed there are several classes that implement interfaces such as Iterator or Countable without adding the return types from the corresponding interfaces. Based on the existing #[\ReturnTypeWillChange] attribute I understand that the return type cannot be declared for an overriding method due to PHP cross-version compatibility concerns. But I don't understand why and for how long is the lack of return types still supported in the latest version of this package.

e.g. Laminas\Db\Sql\Join

Thank you!