[Form] Restrict types for the `label` option for form fields

This is a follow up of https://github.com/symfony/symfony/pull/41717. There we allowed `TranslatableMessage::class` next to `string` and `null` for the form field option `help`. The form field option `label` does currently not have any restrictions for allowed types.

So the question would be: Should we restrict the allowed types for `label` to the same as for `help`?