minor #12691 [Validator] add deprecation log to class ApcCache (#12665) (kbsali)

This PR was merged into the 2.7 branch.

Discussion
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[Validator] add deprecation log to class ApcCache (#12665)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12665
| License       | MIT
| Doc PR        |

This adds depreciation note for the Symfony\Component\Validator\Mapping\Cache\ApcCache class.

Commits
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881ed1e [Validator] add deprecation log (#12665)

Author: fabpot

Mapping/Cache/ApcCache.php

@@ -23,6 +23,8 @@ class ApcCache implements CacheInterface
 
     public function __construct($prefix)
     {
+        trigger_error('The Symfony\Component\Validator\Mapping\Cache\ApcCache class is deprecated since version 2.5 and will be removed in 3.0. Use DoctrineCache with Doctrine\Common\Cache\ApcCache instead.', E_USER_DEPRECATED);
+
         if (!extension_loaded('apc')) {
             throw new \RuntimeException('Unable to use ApcCache to cache validator mappings as APC is not enabled.');
         }