…ator in EsmtpTransport (wikando-ck)

This PR was merged into the 4.4 branch.

Discussion
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[Mailer] Fix string-cast of exceptions thrown by authenticator in EsmtpTransport

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #45308
| License       | MIT

Replace an exception that was being cast to string with a call to `getMessage()`.
This prevents information about the system leaking into the exception, (files/directories, stack trace).

Additionally (see https://symfony.com/releases):

I wasn't able to inject mocked authenticators, as they are instantiated in the constructor.
This makes unit testing very hard.
I was able to reproduce the problem with this test, but it will hit a real smtp server and should not be added to a testsuite.

```php
   // EsmtpTransportTest.php

    public function testAuthExceptionContainsNoStackTrace()
    {
        $transport = new EsmtpTransport('smtp.ionos.de', 465, true);
        $transport->setUsername("test");
        $transport->setPassword("test");
        $message = new Message();
        $message->setHeaders(new Headers(
            new MailboxListHeader("From", [Address::create('[email protected]')]),
            new MailboxListHeader("To", [Address::create('[email protected]')]))
        );
        try {
            $transport->send($message);
            $this->fail("Expected TransportException");
        } catch (TransportExceptionInterface $e) {
            $this->assertStringNotContainsString("Stack trace:", $e->getMessage());
            $this->assertStringNotContainsString('Symfony\Component\Mailer\Transport\Smtp', $e->getMessage());
        }
    }
```

Commits
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7310ea5 [Mailer] Fix string-cast of exceptions thrown by authenticator in EsmtpTransport