minor symfony#52714 [Serializer] Remove incompatible type declaration with PHP 7.2 (alexandre-daubois)

This PR was merged into the 5.4 branch.

Discussion
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[Serializer] Remove incompatible type declaration with PHP 7.2

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

Fixes CI

Commits
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06d6827 [Serializer] Remove incompatible type declaration with PHP 7.2

Author: nicolas-grekas

src/Symfony/Component/Serializer/Tests/Normalizer/PropertyNormalizerTest.php

@@ -596,10 +596,10 @@ interface PropertyDummyInterface
 
 class PropertyDiscriminatedDummyOne implements PropertyDummyInterface
 {
-    public string $url = 'URL_ONE';
+    public $url = 'URL_ONE';
 }
 
 class PropertyDiscriminatedDummyTwo implements PropertyDummyInterface
 {
-    public string $url = 'URL_TWO';
+    public $url = 'URL_TWO';
 }