minor symfony#57972 [FrameworkBundle] Re-remove redundant name attribute from default_context (HypeMC)

This PR was merged into the 7.1 branch.

Discussion
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[FrameworkBundle] Re-remove redundant name attribute from `default_context`

| Q             | A
| ------------- | ---
| Branch?       | 7.1
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

This was originally removed in symfony#53657 but was later reintroduced in v7.1.2, likely by accident, in the following merge commit: symfony@67ac926.

Commits
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83fe767 [FrameworkBundle] Re-remove redundant name attribute from `default_context`

Author: nicolas-grekas

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -1118,7 +1118,6 @@ private function addSerializerSection(ArrayNodeDefinition $rootNode, callable $e
                         ->end()
                         ->arrayNode('default_context')
                             ->normalizeKeys(false)
-                            ->useAttributeAsKey('name')
                             ->validate()
                                 ->ifTrue(fn () => $this->debug && class_exists(JsonParser::class))
                                 ->then(fn (array $v) => $v + [JsonDecode::DETAILED_ERROR_MESSAGES => true])