[Validator] Simplified usage of the Callback constraint

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | fixes #2480, replaces #3939
| License       | MIT
| Doc PR        | symfony/symfony-docs#3012