[FrameworkBundle] Fixed validator factory definition when the Validator API is "auto" for PHP < 5.3.9

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11580
| License       | MIT
| Doc PR        | -
this is why I said to put the default value to ``2.4`` or ``2.5-bc`` depending on the PHP version, not to ``auto``.

And currently, setting ``auto`` explicitly is supported. As your change does not allow it anymore, it is a BC break and cannot be merged. This is why I'm saying it must be handled as well in the normalization part.