[Security] renamed Constraint namespace to Constraints for validator classes in order to be consistent with the whole current validator API.
merged branch hhamon/user_password_constraints_namespace (PR #6960)

This PR was merged into the 2.2 branch.

Commits
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35b62ac [Security] renamed Constraint namespace to Constraints for validator classes in order to be consistent with the whole current validator API.

Discussion
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[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        | -