[Security] decoupled UserPassword constraint from SecurityBundle

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes (for people already using the ``UserPassword`` constraint class in their code)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6889
| License       | MIT
| Doc PR        | symfony/symfony-docs#123