[DoctrineBridge] Changed UniqueEntityValidator to use the 2.5 Validation API

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11578
| License       | MIT
| Doc PR        | -
an even easier way for bundles (avoiding to implement the switching logic in 4 bundles, keeping in mind the case of older Symfony without the class and version param) would be to detect the version being used in the validator and switching the way to add the violation, if possible. This way, we could use a single class with a small switch in it.