[Form] [Hackday] [2.7] Add a deprecation note about VirtualFormAwareIterator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12604
| License       | MIT
| Doc PR        | 
@nicolas-grekas putting it when loading the file does not trigger a warning each time the class is instantiated (and could trigger it when not needed). We need to trigger the error only when the deprecated feature is used (which may not even be at instantiation time if it is always instantiated for BC reasons)