[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        | 
why not throwing the trigger_error in the constructor and not after the namespace declaration, like: https://github.com/symfony/symfony/pull/12739/files ?