feature #35815 [Validator] Allow Sequentially constraints on classes + target guards (ogizanagi)

This PR was merged into the 5.1-dev branch.

Discussion
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[Validator] Allow Sequentially constraints on classes + target guards

| Q             | A
| ------------- | ---
| Branch?       | master <!-- see below -->
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | N/A <!-- prefix each issue number with "Fix #", if any -->
| License       | MIT
| Doc PR        | todo in symfony/symfony-docs#13206 if not merged yet

There is no reason to limit this constraint to properties, so let's add classes as targets.

Additionally, let's ensure embedded constraints matches allowed targets too.

Commits
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1e02a96286 [Validator] Allow Sequentially constraints on classes

Author: nicolas-grekas

Constraints/Sequentially.php

@@ -16,7 +16,7 @@
  * Validation for the nested constraints collection will stop at first violation.
  *
  * @Annotation
- * @Target({"PROPERTY", "METHOD", "ANNOTATION"})
+ * @Target({"CLASS", "PROPERTY", "METHOD", "ANNOTATION"})
  *
  * @author Maxime Steinhausser <maxime.steinhausser@gmail.com>
  */
@@ -38,4 +38,9 @@ protected function getCompositeOption()
     {
         return 'constraints';
     }
+
+    public function getTargets()
+    {
+        return [self::CLASS_CONSTRAINT, self::PROPERTY_CONSTRAINT];
+    }
 }