minor symfony#24701 Config: mark builder property deprecated (bburnichon)

This PR was squashed before being merged into the 3.4 branch (closes symfony#24701).

Discussion
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Config: mark builder property deprecated

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        |

`builder` protected property is not used at all in class. Mark as deprecated to enable removing asap. This is to avoid keeping dead code up to symfony `5.0` and still having this dead code in the `4.4` LTS version

Commits
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206fb74 Config: mark builder property deprecated

Author: nicolas-grekas

UPGRADE-3.4.md

@@ -1,6 +1,11 @@
 UPGRADE FROM 3.3 to 3.4
 =======================
 
+Config
+------
+
+ * Using protected builder property of TreeBuilder is deprecated and property will be removed in 4.0.
+
 DependencyInjection
 -------------------
 

src/Symfony/Component/Config/Definition/Builder/TreeBuilder.php

@@ -22,6 +22,10 @@ class TreeBuilder implements NodeParentInterface
 {
     protected $tree;
     protected $root;
+
+    /**
+     * @deprecated since 3.4. To be removed in 4.0
+     */
     protected $builder;
 
     /**