bug #867 Fixed the DateTime constraint deprecation (javiereguiluz)

This PR was merged into the master branch.

Discussion
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Fixed the DateTime constraint deprecation

This is a deprecation introduced by the upgrade to Symfony 4.2 in #865.

```
3x: Validating a \DateTimeInterface with "Symfony\Component\Validator\Constraints\DateTime" is deprecated since version 4.2. Use "Symfony\Component\Validator\Constraints\Type" instead or remove the constraint if the underlying model is already type hinted to \DateTimeInterface.
    1x in BlogControllerTest::testAdminNewPost from App\Tests\Controller\Admin
    1x in BlogControllerTest::testAdminEditPost from App\Tests\Controller\Admin
    1x in BlogControllerTest::testNewComment from App\Tests\Controller
```

Commits
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aec924a Fixed the DateTime constraint deprecation

Author: javiereguiluz

src/Entity/Comment.php

@@ -64,7 +64,6 @@ class Comment
      * @var \DateTime
      *
      * @ORM\Column(type="datetime")
-     * @Assert\DateTime
      */
     private $publishedAt;
 

src/Entity/Post.php

@@ -87,7 +87,6 @@ class Post
      * @var \DateTime
      *
      * @ORM\Column(type="datetime")
-     * @Assert\DateTime
      */
     private $publishedAt;