minor #11062 UPGRADE-2.5.md: Yaml component change in 2.5 (nikita2206)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #11062).

Discussion
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UPGRADE-2.5.md: Yaml component change in 2.5

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

We had a very hard to find bug when I was trying to upgrading symfony from 2.4 to 2.5, I think it's worth mentioning in the UPGRADE file. I understand we shouldn't have had duplicated keys in yaml but that was it...
BTW maybe we should raise an error in cases like this?

PS I figured I shouldn't include the PR template with table here because it's just a UPGRADE-* file change, I will add it if you say

Commits
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3931723 Yaml component change in 2.5

UPGRADE-2.5.md

@@ -226,3 +226,26 @@ Validator
        ->getValidator();
    ```
 
+
+Yaml Component
+--------------
+
+ * The way Yaml handles duplicate keys in an array was changed from `rewrite with the
+   last element` behavior to ignoring all the elements with the same key after the first one.
+
+   Example:
+
+   ```
+   parentElement:
+       firstChild: foo
+       secondChild: 123
+       firstChild: bar
+   ```
+
+   Before:
+
+   This would be parsed in an array like this: `["parentElement" => ["firstChild" => "bar", "secondChild" => 123]]`
+
+   After:
+
+   The first value is used: `["parentElement" => ["firstChild" => "foo", "secondChild" => 123]]`