Symfony version(s) affected: 5.1.5

Description
I'm using Symfony Mailer with Mailgun API integration. When sending an e-mail and the Mailgun API is not properly configured (e.g. wrong API key), I get this exception:

Symfony\Component\HttpClient\Exception\JsonException
"Response content-type is "text/html; charset=utf-8" while a JSON-compatible one was expected for "https://api.mailgun.net/v3/sandbox562c79333e4345ff9c02a544d6076a4d.mailgun.org/messages"."

As this exception is not caught properly, also prevents the failover mechanism to work (if configured).

How to reproduce
Just configure a wrong API key in the mailer DNS. Mailgun will reply with a text/html reponse and Forbidden as content. This will cause the exception
Similar HTML responses are obtained by using a non allowed method or by using a non existing domain.

Possible Solution
This was previously fixed with this PR: #33432 by removing the unsafe call to Response::toArray, but somehow got re-introduced (not sure in which Symfony version).

I believe this can be fixed by moving the following line inside the nested if, 2 lines below: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Mailer/Bridge/Mailgun/Transport/MailgunApiTransport.php#L67

On line 76, we would need to find another way to retrieve the message ID. I believe it's OK to re-parse the response as that would be anyway exclusive (one for success, another for failures).

Additional context
You can quickly verify that Mailgun replies with a content type text/html; charset=utf-8 with this cURL snippet:

curl -v --location --request POST 'https://api.mailgun.net/v3/anydomain.com/messages' --header 'Accept: application/json'