minor #39463 [Mailer] [Amazon] Ensure "ext-simplexml" is loaded if SesApiTransport...

minor #39463 [Mailer] [Amazon] Ensure "ext-simplexml" is loaded if SesApiTransport is used (OskarStark)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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[Mailer] [Amazon] Ensure "ext-simplexml" is loaded if SesApiTransport is used

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | ---
| License       | MIT
| Doc PR        | ---

For reference:
https://github.com/symfony/symfony/blob/150d8506e425212bbdc0e75b7d5fc4e6710f0852/src/Symfony/Component/Mailer/Bridge/Amazon/Transport/SesApiTransport.php#L66-L69

Commits
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a40d1375f9 [Mailer] [Amazon] Ensure "ext-simplexml" is loaded if SesApiTransport is used