[Validator] Moved constraints Optional and Required to the Constraints\ namespace
merged branch bschussek/move-existence-constraints (PR #7701)

This PR was merged into the master branch.

Discussion
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[Validator] Moved constraints Optional and Required to the Constraints\ namespace

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#2512

Commits
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a868048 [Validator] Moved constraints Optional and Required to the Constraints\ namespace