minor symfony#47951 Fix duplicate heading in upgrade notes for Serializer component (jt2k)

This PR was merged into the 4.4 branch.

Discussion
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Fix duplicate heading in upgrade notes for Serializer component

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| License       | MIT

Minor fix to the 5.0 upgrade notes document to remove the second heading for the Serializer component and merge the two sections together.

Commits
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8832d02 Merge upgrade notes for Serializer component

Author: fabpot

UPGRADE-5.0.md

@@ -525,10 +525,6 @@ Serializer
    were removed, use the default context instead.
  * The `AbstractNormalizer::handleCircularReference()` method has two new `$format` and `$context` arguments.
  * Removed support for instantiating a `DataUriNormalizer` with a default MIME type guesser when the `symfony/mime` component isn't installed.
-
-Serializer
-----------
-
  * Removed the `XmlEncoder::TYPE_CASE_ATTRIBUTES` constant. Use `XmlEncoder::TYPE_CAST_ATTRIBUTES` instead.
 
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