[HttpFoundation] deprecates FlashBag::getIterator() method

This PR does not fix #8294 but this issue is not fixable. The `FlashBag::getIterator()` method actualy does not make any sense and is confusing. It should be removed. I just added a `@deprecated` tag and `Will be removed in 3.0.` message because removing it now would introduce a BC break. I guess issue #8294 can be closed after merge as it make not more sense than the incriminated method.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8294