minor #25565 [2.7][DX] Use constant message contextualisation for deprecations (romainneutron)

This PR was merged into the 2.7 branch.

Discussion
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[2.7][DX] Use constant message contextualisation for deprecations

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

Since many projects are using this mechanism for deprecating feature, it is sometimes difficult to understand which vendor triggers a deprecation.
Sometimes we're using `since Symfony x.y` format, sometimes we are using `since x.y`. I propose to always use `since Symfony x.y` format.

Commits
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c2338cbd7a [2.7][DX] Use constant message contextualisation for deprecations

Author: fabpot

Session/Flash/FlashBag.php

@@ -161,7 +161,7 @@ public function clear()
      */
     public function getIterator()
     {
-        @trigger_error('The '.__METHOD__.' method is deprecated since version 2.4 and will be removed in 3.0.', E_USER_DEPRECATED);
+        @trigger_error('The '.__METHOD__.' method is deprecated since Symfony 2.4 and will be removed in 3.0.', E_USER_DEPRECATED);
 
         return new \ArrayIterator($this->all());
     }