bug symfony#57621 [Mailer] force HTTP 1.1 for Mailgun API requests (xabbuh)

This PR was merged into the 5.4 branch.

Discussion
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[Mailer]  force HTTP 1.1 for Mailgun API requests

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | Fix symfony#57544, Fix symfony#57569 (comment)
| License       | MIT

Commits
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0bf9e72 force HTTP 1.1 for Mailgun API requests

Author: fabpot

src/Symfony/Component/Mailer/Bridge/Mailgun/Transport/MailgunApiTransport.php

@@ -61,6 +61,7 @@ protected function doSendApi(SentMessage $sentMessage, Email $email, Envelope $e
 
         $endpoint = sprintf('%s/v3/%s/messages', $this->getEndpoint(), urlencode($this->domain));
         $response = $this->client->request('POST', 'https://'.$endpoint, [
+            'http_version' => '1.1',
             'auth_basic' => 'api:'.$this->key,
             'headers' => $headers,
             'body' => $body->bodyToIterable(),