[Form][Validator] All index items after children are to be considered grand-children when resolving ViolationPath

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | unsure, see note below
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11458
| License       | MIT
| Doc PR        | -

#### Possible BC Break
The old behavior had unit test cases specifically testing the case of a grand-children form. However, this behavior is not documented anywhere and the fix seems to have no adverse effects on form validation. `Symfony\Component\Form\FormInterface` implements `ArrayAccess`, therefore, semantically speaking, `children[direct_child].children[grand_children]` and `children[direct_child][grand_children]` are equivalent. `offsetGet` is expected to fetch an element from `children`. I do not see why both were not considered equivalent when resolving the ViolationPath.

This commit will indeed change how some errors are mapped. However since the old mapping is (in my opinion) a bug...