bug #31423 Fix compat with older versions of PHP (fabpot)

This PR was merged into the 4.3-dev branch.

Discussion
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Fix compat with older versions of PHP

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

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Commits
-------

07b9af2eff fixed compat with older versions of PHP

src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php

@@ -133,7 +133,10 @@ class Deprecation
         return $this->originMethod;
     }
 
-    public function getMessage(): string
+    /**
+     * @return string
+     */
+    public function getMessage()
     {
         return $this->message;
     }