[Security] renamed Constraint namespace to Constraints for validator classes in order to be consistent with the whole current validator API.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes (for people already using the ``UserPassword`` constraint class in their code)
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -