minor symfony#18588 [Serializer] Add deprecation notices (theofidry)

This PR was squashed before being merged into the 3.1-dev branch (closes symfony#18588).

Discussion
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[Serializer] Add deprecation notices

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | symfony#17140
| License       | MIT
| Doc PR        | none

As discussed in symfony#18583, add deprecation notices to `master` instead.

Commits
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d3063ed [Serializer] Add deprecation notices

Author: nicolas-grekas

src/Symfony/Component/Serializer/Serializer.php

@@ -56,11 +56,15 @@ class Serializer implements SerializerInterface, NormalizerInterface, Denormaliz
 
     /**
      * @var array
+     *
+     * @deprecated since 3.1 will be removed in 4.0
      */
     protected $normalizerCache = array();
 
     /**
      * @var array
+     *
+     * @deprecated since 3.1 will be removed in 4.0
      */
     protected $denormalizerCache = array();